942 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 215. 



Fig. 216), or liorizontally (Fig. 217), and two small apertures give addi- 

 tional facilities for varying the effects (Fig. 218). The apparatus was 

 constructed for Mr. Tighlmann by Messrs. Queen, of Philadelphia, 

 U.S.A., and can be commended for its simplicity. The milling on 

 the moving cylinder should, however, project much more, so as to 

 present a better grip for the adjustment. The cylinder should also 

 be lined with cloth to diminish the friction 

 in moving. The vertical illuminator is 

 seldom used except with high-power ob- 

 jectives, and it requires very delicate 

 manipulation ; unless, therefore, the dia- 

 phragm can be moved with a light touch, 

 the image is apt to make bewildering 

 excursions — especially when the apparatus 

 is used with a Microscope focussing at the 

 nose-piece. One of the principal considera- 

 tions to be held in view in the application 

 of diaphragms to illuminating devices 

 should be to enable the observer to watch 

 the effect produced during the actual 

 movement of the diajihragm. The accurate discrimination of the 

 more difficult images with high powers is so largely dependent on 



Fig. 216. 



Fig. 217. 



Fig. 218. 



the convenient arrangement of the means for slightly varying the 

 illumination, that we should consider any device defective in so far 

 as it could not be controlled by the observer without removing his 

 eye from the eye-piece. 



Mr. Tighlmann's new diaphragm appears to us capable of de- 

 veloping the power of the vertical illuminator beyond the point 

 hitherto attained. 



New Homogeneous-immersion 1-12-inch of 1'43 N. A. — Mr. T. 



Powell has constructed a homogeneous-immersion ^^^"i^ich objective 

 of 1'43 N. A. without mounting the front lens on thin glass as 

 described in this Journal, vol. iii. (1880) pp. 884 and 1050. The 

 mounting of the front lens on a plate of glass of only • 003 in. thick- 

 ness is obviously not a very secure method, whilst it necessitates 

 special care in the use of the objective. 



Fluid for Homogeneous-immersion. — Prof. Abbe has tested the 

 chloral hydrate in crusts suggested (with glycerine) as a fluid for 



