126 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 17. 



viding convenient centering motions with whicli small instruments 

 are not usually supplied by opticians. The optical part of a -^^ ob- 

 jective then forms an excellent condenser ; for this purpose it should 

 be fitted with the shortest possible adapter, so that diaphragms may 

 be used close beneath the back lens. 



Mr. Nelson has also suggested the application of diaphragms 

 beneath such a condenser in an annular disk-fitting swinging sideways 



on a pivot, the disk having a ledge 

 within, on which one, two, or three 

 diaphragms of various shapes may 

 be placed together, and rotated by a 

 milled edge (shown in the figure pro- 

 jecting beneath with shaped handle 

 serving to move the disk out of the 

 axis to change the diaphragms). The 

 practical management of the dia- 

 phragms is obviously of the first 

 importance in obtaining variety of 

 eifects of light with a condenser in 

 the axis. Mr. Nelson has found the 

 most useful series of diaphragms to 

 be those represented in Fig. 18, in which a may be regarded as a type 

 shape for one jiencil of light, and h for two — at right angles. The 

 superposition of stops like c will cut off more or less of the central 

 light, while d will cut off more or less of the peripheral zone ; e is 



Fig. 18. 



a combination in which the square opening is intended to utilize the 

 most oblique pencil required for the resolution of very fine lines, &c., 

 whilst the small circular aperture is calculated to give a beam of light 

 for the resolution of less difficult markings on the same object at right 

 angles — for instance, on Surirella gemma. It is advisable to have a 

 variety of sizes of c and d, as upon these depend most of the difficult 

 resolution to be obtained by using the condenser in the axis. 



Of course the above arrangement for diaphragms can be applied to 

 any form of condenser, and is not confined to the centering nose- 

 piece. 



May all's Spiral Diaphragm for Oblique Illumination. — At the 

 Society's December meeting, Mr. J. Mayall, jun., described a spiral 

 diaphragm, which he had devised as a convenient means of obtaining 

 oblique illumination in connection with high-angled condensers to be 

 used in the axis of the Microscope. 



If a slot diaphragm (as shown in Fig. 19) be fixed close beneath 

 the large lens of a high-angled condenser, such as those lately con- 

 structed by Powell and Lealand, and by Zeiss, the rotation under 



