ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 103 



when the motion was reversed, notwithstanding that the opposing spring 

 was a strong one, a " single speed " lever motion was tried, the ratio of 

 the arms of the lever being 8 to 1, the fine adjustment screw having 

 100 threads to the inch, with an opposing spring strong enough to 

 require 7 lb. to move it. This was found to require a weight of only 

 £ oz. to turn the head of the fine adjustment screw, and to work in a 

 perfectly satisfactory manner. 



(2) Eye-pieces and [Objectives. 



Spencer Objective.*— F. J. Keeley describes a Microscope objec- 

 tive of I inch focus made in 1860 by Charles A. Spencer. It was 

 recently necessary to take apart the back system for re-balsaming, when 

 it was found to consist of five lenses, three of which were convex and 

 two concave. One of these proved, on examination with polarized light, 

 to be fluorite. This objective is historically interesting as illustrating 

 the complex nature of the corrections adopted by Spencer at so early a 

 date, as well as confirming the previous reports that he had appreciated 

 the possibilities connected with the use of fluorite in securing superior 

 colour corrections, and employed it for the purpose twenty years before it 

 came into use abroad. The objective has an aperture of 142 to \^2 

 degrees, according to position of the collar adjustment, which acts by 

 the movement of the back systems, and it is unusually well corrected 

 for colour. It resolves Pleurosigma angulatum sharply into dots with 

 central light from mirror, and with oblique illumination resolves mark- 

 ings 76,000 to the inch. 



H. — Construction of aplanatic combinations of lenses, with or without achromatism. 



English Mechanic, lxxx. (1904) pp. 252-3, 321-2, 340, 406-8. 

 Merlin, A. A. C. — Microscopical high powers and deep eye-pieces. 



[The writer says that if a given objective capable of affording a really clear, 

 brilliant, and well-contrasted image under a x 12 ocular when a large solid 

 illuminating cone is used, it may be employed, if necessary, in conjunction 

 with the deepest eye-pieces, so as to give results just as satisfactory as would 

 be attainable with a higher power objective of equal N.A. combined with 

 a shallow eye-piece. Tom. cit., p. 455. 



Vill agio— Ditto. Tom. cit., p. 3S4. 



(4) Photomicrography. 



Photomicrography with the Aid of Ultra-Violet Light.f — Text- 

 books of science, as a rule, explain microscopic vision with the aid of 

 rays. This elementary explanation does not fix any limit to the possible 

 magnification, but as long as we have not to deal with dimensions which 

 are comparable to the wave-length of light, it does not bring us into 

 conflict with observed facts. But we reach the limit of resolution when 

 the distances between the lines of the object are less than half a wave- 

 length of the light with which we illuminate the object. The theory 

 which Helmholtz advanced for self-luminous objects, and Abbe, about 

 the same time, for illuminated objects, regards the microscopic images 

 as diffraction phenomena ; and this theory, some points of which Dr. 

 Glazebrook has recently cleared up, also indicates the way in which 



* Proo. Acad. Nat. Sci., Philadelphia, lvi. (1904) p. 475. 

 t Engineering, lxxviii. (1904) p. 700. 



