254 HOW TO WORK 



to make the rays converge slightly, and the other at a point where 

 it will include all these rays and (in using high powers) the achromatic 

 condenser, so that the lens may fall well within the cone of rays. In 

 employing low powers the object is made to come within the cone 

 of converging rays. The distance of the lamp from the nearest lens 

 to it, is best determined by the quality of the illuminated field, which 

 should be equally bright, nor should the light enter the objective at 

 a greater angle than its own angular aperture. To examine the 

 image thrown on the ground glass of the camera, Mr. Shadbolt used 

 a Ramsden's positive eye-piece. 



Mr. Legg, in 1859, made use of artificial light from a camphine 

 lamp, concentrating the diverging rays by a two-inch bull's-eye lens 

 placed near to the source of light, and a second bull's-eye lens about 

 three inches in diameter at a distance of an inch from the first, by 

 which, with the 2-3rds and 4-ioths object-glasses, he could obtain 

 images at 3 feet, in periods varying from 3 to 10 minutes. 



Mr. Parry, in making use of artificial light, placed a plano-convex 

 of ii inch focus with its plane side towards the object about one 

 inch from it, and four or five inches from an argand gas burner. (The 

 light from an argand paraffin lamp is preferable to gas.) To increase 

 the flatness of the field, he fixes behind the posterior lens of the 

 i -inch combination, an achromatic stereoscope camera lens with its 

 flat surface towards the objective. The advantage of the brilliant 

 light produced by the combustion of magnesium wire is referred 

 to in p. "2 7 5. 



334. Of Focussing. Much care is required in focussing. A plan 

 adopted by some is to use a simple lens set as a watchmaker's loupe 

 in a card or wooden tube of such a length, that when placed at the 

 near surface of the ground-glass screen, the focus of the lens exactly 

 corresponds to the opposite or ground side. Others employ an 

 ordinary photographic focussing eye-piece. The best is the positive 

 eye-piece, for should the others not be truly set, there is danger of 

 the focus catching the image either before or behind the screen, 

 unless some form of compound microscope be employed, with its 

 focus set to the exact thickness of the screen. 



335. Of the Object-glasses. Each objective, as furnished by our 

 best opticians, is generally sent out not as a photographic object-glass, 

 but as a microscopic objective, and so skilfully have errors which 

 arise from the thickness of the thin glass cover and non-achromaticity 

 of the eye-piece been compensated for, that the illuminated field is 

 without sensible colour, and the edges of objects destitute of chro- 

 matic fringes. To accomplish this, the objective is left what is termed 

 " over corrected." 



