138 NOTES ON THE CONSTRUCTION AND DESIGN 



of the " super-microscope " arrangement, by which the 

 image formed by one objective is magnified by a second 

 objective. 



(f) The Vertical lllumnidtor. — Whilst the prism form has the 



advantage for visual work of causing much less loss of 

 light than the transparent plate, it is unsuitable for high 

 powers, on account of the fact that it only uses one-half 

 of the aperture of the objective, and is consequently liable 

 to produce false images of fine structures. The same 

 objection applies to silvered half-discs or other similar 

 devices. The Beck or transparent illuminator is the only 

 suitable form for photographic work at any but low mag- 

 nifications. The mistake is very commonly made of fitting 

 a small cover glass, which only imperfectly covers the back 

 lens of the objective, into such illuminators. A plate of 

 larger size should be used. Moreover, cover glasses are not 

 accurately flat, and have no advantage except cheapness 

 and thinness. A large, optically worked plate is used in 

 the Conrady-Watson illuminator and in the Jackson and 

 Blount microscope. The writer has found the thin, square 

 plates used for counting blood corpuscles very suitable, 

 being sufficiently flat and so thin as not to produce doubling 

 of the image. The plate should be capable of at least 

 partial rotation, and should have a sufficiently large milled 

 head to allow of delicate adjustment. Vertical illuminators 

 often leave much to be desired in regard to mechanical 

 construction. 



The inverted or Le Chatelier type of microscope calls 

 for a different form of illuminator. As usually constructed, 

 the numerous reflections required tend to injure the defini- 

 tion of the image and to cause loss of light. To a great 

 extent this might be obviated by better optical workman- 

 ship, the prism being made in one piece with accurately 

 ground faces, as in the modern range finder. The possi- 

 bilities of new optical arrangements for illumination are 

 not exhausted. 



(g) The Objectives. — It is now generally agreed that short 



mounts are to be preferred for metallographic objec- 

 tives. A high numerical aperture is necessary for the 

 highest powers. Apochromats are usually recommended for 

 the medium and high powers, but such objectives are 

 commonly deficient in flatness of field, a very desirable 

 quality in metallographic work, and it may be questioned 

 whether good achromats, giving flat fields, are not to be 

 preferred for photographic purposes. It is usual to insert 

 a colour screen when making such photographs, and now 

 that screens which transmit so narrow a band of the spec- 

 trum that they may be regarded as practically mono- 

 chromatic are obtainable, it seems of less importance that 

 the colour correction of the objectives should be perfect. 

 Oil immersion objectives are, of course, necessary for the 

 highest magnifications. 



