ON THE CONSTRUCTION AND APPLICATION OF A TESTING CHART.

By RICHARD TROTTER JEFFCOTT.

O build and install such an apparatus as our title suggests was a problem before me in the early months of this year. Perhaps the most difficult point to be overcome was the adapting of such an instrument to our store conditions. Taking

into consideration the many problems confronting one, the mode of construction and the application of the chart to a commercial enterprise will prove interesting reading.

The fact that our sales of high-grade lenses had increased to such an extent that to present to intending purchasers a superior method of determining what particular lens would best suit his requirements was ever before us. The lens buyer had only such description and data as the lens catalogues gave him. He then might possibly select a lens and give it, in his own way, a thorough test, yet hardly with the ready understanding of the lens's capabilities. The lens chart would give to him an opportunity to test all makes of lenses along the same line and under precisely the same conditions, and, further, give him a complete record of what he might expect from each and every lens tested.

Where one was buying a second-hand lens of perhaps unknown specifications, we would be in a position to demonstrate the best that particular lens would perform on the chart.

Having before me the above advantages that such an apparatus would give to the public, and, further, having collected data from many sources, together with special trips to New York and Rochester, I was now in position to put the idea into practical use.

It may, perhaps, be well to mention here at the outset that criticisms of our idea were welcomed and further given every consideration, yet if we have erred in building an apparatus heavier and more solid than our needs required, we trust our judgment suggested that strength and solidity guarded against our worst enemy-warping.

Rearranging our store fixtures provided a space 3 feet by 13 feet for the specially constructed platform to support the entire apparatus.

Detail drawings are herewith presented, giving, in addition, figures indicating sizes and heights, and a descriptive outline of construction may be in order. Our platform was built

FRONT VIEW OF APPARATUS FOR DETERMINING DEPTH OF FOCUS.

about 33% inches above the floor line, the frame was made of 3 x 4 inch joists and braced every two feet. The flooring was of Georgia pine, all being of the best selected stock, thoroughly dried and braced against warping. Attached to the outside of the platform a steel track was secured, same being 3% inch higher than the platform proper. This was used for the mov