this case a section from near the middle of an X-chromosome of D. melanogaster appears to have broken loose and attached itself to the left-hand end of a normal X-chromosome. Weinstein' has shown that such an occurrence might lead to a change of sequence of identical loci such as is here reported. If we suppose that the simulans third chromosome was originally constituted as is that of melanogaster, the situation as we now find it may be supposed to have arisen as follows: A section, including the peach locus, broke loose and attached itself near the right-hand end of a normal third chromosome. After this condition had become established the peach locus near the middle of the chromosome mutated or became "deficient," so that in effect the peach locus was moved to the right end of the chromosome. Such an interpretation will account for the observed facts.8

There is, however, another possible method whereby the same result might be supposed to have been brought about, viz., by the simple inversion of a section of a normal chromosome. Such an accident seems not unlikely to occur at the stage of crossing over. If we suppose a chromosome to occasionally have a "buckle" at a crossing over point, it is conceivable that crossing over might be followed by fusion of the broken ends in such a way as to bring about an inversion of a section of chromosome. Either of the two suppositions discussed will account for the observed results, but they should lead to different relations for other loci in the same chromosome; it is hoped that further work will lead to the discovery of additional parallel mutations, so that the maps may be studied in more detail. If an inversion of the kind suggested above occurred within a species, then individuals bearing one normal chromosome and one chromosome with an inverted section would probably show no crossing over in the region in question, since it seems probable that synapsis in this region would be abnormal or absent. It would also be not surprising if crossing over in adjacent regions was decreased. But individuals homozygous for the inverted section would be expected to show free crossing over again, since there should now be no difficulty at synapsis.

The relations indicated are those that have actually been found in the cases of the two "crossover genes" in melanogaster known as C and CII.10 These "genes" both cause, in individuals heterozygous for them, the disappearance of crossing over in the immediate regions where the "genes" themselves lie, and a considerable reduction of crossing over in neighboring regions. In individuals homozygous for either of these "genes," however, the percentage of crossing over rises to (or beyond) that found in "normal" individuals. Experiments are now under way in an attempt to determine if these "genes" are really simply inverted chromosome sections, but it will probably be a long task to definitely settle the matter.

The demonstration of a change in sequence of identical loci that is here reported makes the identification of parallel mutations in species that cannot be crossed even more difficult than it has previously seemed; for identity of sequence in a group of identical loci now appears not to be necessarily expected.

1 Contribution from the Carnegie Institution of Washington.

2 Sturtevant, A. H., Genetics, 6, 1921 (63, 179).

3 Discovered by Prof. T. H. Morgan.

• Discovered by Dr. C. B. Bridges.

5 This map is based on the more extensive one published by Bridges in these PRO

CEEDINGS.

6

• Bridges, C. B., J. Gen. Physiol., 1, 1919 (645).

7 Weinstein, A., these PROCEEDINGS, 6, 1921 (625).

8 It is, of course, possible to invert this interpretation by supposing the simulans situation to be the original one.

9 Muller, H. J., Amer. Nat., 50, 1916 (103, 284, 350, 421), and Sturtevant, A. H., Carnegie Inst. Wash. Publ., No. 278, 1919 (305).

10 Sturtevant, A. H., these PROCEEDINGS, 3, 1917 (555), and Carnegie Inst. Wash. Publ., No. 278, 1919 (305).

A REMEASUREMENT OF THE RADIATION CONSTANT, h, BY MEANS OF X-RAYS

BY WILLIAM DUANE, H. H. PALMER AND CHI-SUN YEH

JEFFERSON PHYSICAL LABORATORY, HARVARD UNIVERSITY

Communicated July 6, 1921

Since the discovery of the fact that the continuous X-ray spectrum has a short wave-length limit, which obeys the quantum law, a number of experimentors have used this phenomenon to determine the value of h.2 In its application to X-rays the quantum law may be expressed by the equation (1)

Ve = hv,

where V represents the maximum difference of potential in the X-ray tube through which the electrons fall, e, the charge carried by each electron, v, the frequency of vibration corresponding to the short wave-length limit of the spectrum, and h, Planck's action constant. Evidently a measurement of V and gives us the ratio of h to e, and from this we get h, if we suppose e to be given by other experiments. Blake and Duane3

made the most accurate measurement of h in our laboratory. Using Millikan's value of e 4.774X10-10 they obtained the value h = 6.555 X10-27.

=

The object of the research reported in this note is to increase the accuracy of the measurement of h. We use a new and somewhat improved spectrometer and a new calcite crystal. The X-ray tube contains a tungsten target and a Coolidge cathode. A side arm attached to the tube extends out toward the spectrometer and carries at its outer end a thin mica window. The increased intensity of the X-rays coming through this window enables us to use a narrower spectrometer slit, which reduces the correction that must be made for the slit's width.

As in the previous researches the high tension storage battery supplies the current through the X-ray tube. In the present research we have greatly increased the accuracy of the measurement of the difference of potential applied to the tube. Whereas in the previous researches this difference of potential was compared with the electromotive force of a standard cell through the calibration of several intermediate instruments (an electrostatic voltmeter, an ammeter, and a potentiometer), we now compare the difference of potential directly with the electromotive force of a standard cell by means of the simple potentiometer method. In this way we eliminate the errors in the calibrations of the various instruments. The main circuit in the simple potentiometer consists of a large number of coils of manganin wire having a total resistance of over six million ohms. We use the same precautions in insulating the various circuits as one employs in making measurements of ionization currents. The ratio of the resistances of two sections of the main circuit gives us directly the ratio of the difference of potential to the electromotive force of the standard cell. As the ratio of two resistances can be measured with extreme accuracy, we think that we know the difference of potential applied to our X-ray tube with about the accuracy with which the electromotive force of the standard cell has been determined. We use two unsaturated Weston standard cells, each of which has been tested at the Bureau of Standards, and we compare them with each other from time to time. The certificates from the Bureau give the electromotive forces of the cells to within one part in ten thousand.

The actual drop in potential in an X-ray tube through which the electrons fall differs from the drop in potential measured by the potentiometer by a small amount due to the Volta effect, the current that heats the coil of wire in the Coolidge cathode and the high temperature of the wire. In our experiments the circuits are so connected and the voltage applied to the tube is so high that we may neglect the corrections due to these effects.

In making a measurement of h, one experimenter observes the galvanometer attached to the potentiometer, and by varying a resistance in series with the X-ray tube keeps the difference of potential applied to it constant during the experiment. A second observer measures the current in the spectrometer's ionization chamber. A series of readings is taken on the two sides of the spectrometer's zero near the points at which the continuous X-ray spectrum vanishes. Curves A and B in the figure represent the ionization currents as functions of the angles that fix the positions of the reflecting crystal in one of the experiments. The horizontal portions of the curves correspond to the currents due to natural leak and to stray radiation.

The inclined portions represent the increase in these currents due to the continuous X-ray spectrum. The readings corresponding to the short wave-length limit of the spectrum can be determined to within a few seconds of arc. As indicated in the figure, the difference between these readings on the two sides of the zero gives us twice the glancing angle 0, which must be substituted in the equation

λ

=

2d sin 0 = 6.056 X sin 0 X 10-8cm.

(2)

in order to calculate the shortest wave-length A, in the continuous X-ray spectrum.

A small correction has to be added to the observed value of 0, due to the fact that the source of rays and the slit of the spectrometer are not mathematical lines. The correction for the breadths of the source and slit we determine in two ways. Firstly, we estimate the apparent breadth

of the focal spot as seen from the spectrometer's slit, and then measure the width of the slit by the method described on p. 630 of the paper by Blake and Duane referred to above. Secondly, we measure the breadth of the drop in the ionization curve due to the K critical ionization of bromine. The ionization chamber containes ethylbromide. Curve C in the figure represents this drop in the curve in one of the experiments. The breadth of this drop corresponds within the limits of error of the measurements to the correction to be added to the double angle 20, as determined by the first method. The correction is small. It amounts to less than one part in three hundred.

Incidentally, the measurements we have made of the critical ionization of bromine gives a very accurate measurement of its critical ionization wave-length. As an average we obtained the value

assuming that the grating constant of calcite is 6.056X10-8cm.

We tried to obtain an estimate of the correction for the breadth of the source and the slit by measuring the breadth of a peak on the ionization curve corresponding to a line in the characteristic emission spectrum of the tungsten target. In every case examined, however, the breadth of the peak appeared to be slightly broader than what the measured breadth of the source and slit indicated it should be. This means that the corresponding emission lines have certain finite, intrinsic breadths of their own. If the K critical ionization of bromine has such an intrinsic breadth it is too small to be detected in these experiments. Since from the wave equation,

where c is the velocity of light, we can calculate immediately the maximum frequency in the continuous spectrum. This, together with the difference of potential, V, substituted in equation 1, gives us the ratio of h to e.

The data obtained in four complete measurements of h appear in table 1. Column 2 contains the uncorrected values of 0, column 3 the correc

tion for the breadths of the source and of the slit, and column 4, the corrected values of 0. Column 5 contains the values of the product V sine (which is what we really measure in our experiments). This product depends to a slight extent upon the temperature. The temperature in these experiments