APO CHROMATIC OBJECTIVES 53 



photography rather than for visual work, an arrange- 

 ment still advised by certain makers, but becoming more 

 and more unnecessary as improvements in lens-making 

 are made. With older lenses, not specially corrected to 

 focus simultaneously the actinic and visually brightest 

 rays, it is frequently impossible to get a sharp photo- 

 micrograph unless monochromatic light is used, except 

 by moving the back of the camera, after focussing, 

 by a distance that previous experiments have shown 

 to be necessary. This is obviously a most unsatis- 

 factory method of working and should not be needed 

 with modern objectives, and any lens that does not give 

 a reasonably sharp photograph after careful focussing 

 in ordinary light should be rejected. Colour screens 

 provide a remedy for this state of things by intercepting 

 rays other than those for which the objective is cor- 

 rected before they reach the lens (see p. 161). 



Apochromatic Objectives. It has been mentioned that 

 perfect correction of objectives is unattainable by the 

 combination of ordinary crown and flint glasses, and up 

 to 1886 the difficulties involved in the use of imperfectly 

 corrected lenses were unavoidable. But about that time, 

 as the result of extensive researches, Prof. Abbe and 

 Messrs. Zeiss introduced a new series of objectives, in 

 which they utilised a mineral called Fluorite and new 

 glasses made at the Jena Glass Works. Substances not 

 previously used in glass-making were included in the 

 composition of the new glasses, Phosphoric and Boric 

 oxides in place of Silica, for instance, and by that means 

 materials of optical properties hitherto unattained were 

 produced. A higher refractive index was found in 

 conjunction with a lower dispersion, or vice versa, and 

 a greater proportionality existed in the dispersion of the 

 different parts of the spectrum. The objectives, made 

 as a result of these discoveries, mark a very great 

 advance on anything produced up to that time, and 



