Ruling Test Plates for Microscopic Objectives: Sharpness of Artificial and Natural Points

Abstract

TEST plates for microscopic objectives should consist of alternate opaque and transparent lines approximately of the same width, and placed on a plane surface (not grooves engraved on transparent material). Considerable difficulty has been found in producing such lines when the distance between them is less than about 1/2000 in. They might be made by ruling on thin opaque films, and, so far as opacity is concerned, films of silver or other metals chemically deposited on glass would meet the case; but the intrinsic strength of these films is greater than their adherence to the glass, and the whole of the metal is torn away by the ruling point when the lines are close together. In many trials I have never succeeded in ruling on chemically deposited silver at even 2000 lines per inch, I have found, however, that films of certain aniline colours dried on glass are well adapted for the purpose, their opacity being so intense as to show a fair depth of colour even when the thickness of the film is a very small fraction of a wave-length of visible light. Their adherence also to the glass is greater than their intrinsic strength, and, so far as my experience goes, the limit to the fineness of the lines which may be ruled on them is not reached until the spacing of the lines is less than the thickness of the film.