24 HOW TO WORK 



jnent of the prism, effected by clockwork ; this ray was prevented 

 from passing out of the field. 



. The Diaphragm has been already described in 14. The 

 definition of the structure of a transparent object is often found to 

 be very much clearer when only the more direct and central rays of 

 light from the concave mirror are allowed to pass through it. An 

 excellent contrivance for altering the size of the aperture in the 

 diaphragm has been recently devised by Mr. Collins, fig. 59, pi. XIII. 

 See also 39. 



37. Achromatic Condenser. The illumination of some objects 

 examined with high powers is much improved by causing the light to 

 pass through an achromatic condenser which may consist of an ordi- 

 nary achromatic objective of half or a quarter of an inch focus, 

 arranged in a sliding tube immediately beneath the stage. One of 

 these instruments can be fitted to the student's microscope. 

 Mr. Quekett has adapted a simple lever handle by means of 

 which the right focus is readily obtained, pi. XII, fig. 54. The 

 instrument is not an expensive one, if it be made of a French com- 

 bination. I have often obtained very good illumination suitable 

 for the examination of most tissues without using an achromatic 

 condenser. In working with high powers, however, it is absolutely 

 necessary. 



A Reiner's eye-piece, as already stated, makes a most valuable 

 achromatic condenser, and has been of the most material assistance to 

 me in many of my recent investigations. The observer will find that 

 by stopping off the greater part of the light passing through the con- 

 denser by placing over the upper lens a thin plate with a very 

 small central hole, great advantage results in working with high 

 powers. The hole may be made in a flat piece of thin brass, which 

 is kept in its place by a very slight rim projecting about the twentieth 

 of an inch or less above the top of the condenser. In this way 

 apertures of different sizes may be tried without trouble. My friend 

 Mr. B. Wills Richardson uses stops over the condenser, in which slits 

 and holes are made of peculiar shape, and varying much in position, 

 some allowing only a very small pencil of light to pass at the side. 

 Microscopical Journal, January, 1866, p. TO. 



38. Gillett's Condenser. Mr. Gillett has adapted a diaphragm 

 plate and stops to the achromatic condenser, and there is a beautiful 

 instrument of this kind made by Mr. Ross. Messrs. Powell and 

 Lealand have, however, improved upon it, and brought out a much 

 smaller and more compact condenser, which is attached to their 

 microscope. The Rev. J. B. Reade, to whom we are indebted for 

 many improvements in this direction, has contrived a valuable hemi- 



