1050 



RECORD OF CURRENT RESEARCHES RELATING TO 



When not required for use with the Binocular prism the lenses 

 are screwed to the long adapter A. 



Mr. Wenham many years ago suggested the use cf a very small 

 binocular prism with high-power objectives. The prism was mounted 

 in a special tube and was slipped down the body of the objective 

 almost to touch the back lens. Fig. 120 shows (natural size) an -|- 

 objective (with correction-adjustment) constructed by Messrs. Powell 

 and Lealand on Mr. Wenham's plan, D being the objective complete, 

 and C the tube with the binocular prism. The objective, as will be 

 seen, is shorter than usual. 



Fig. 120. 



Fig. 119. 



The plan first described can be used more effectively with homo- 

 geneous-immersion objectives, as they do not necessarily require 

 correction-adjustment. The body can therefore be much shorter and 

 the back lens almost in contact with the binocular prism. 



Extra Front Lenses to Homogeneous-immersion Objectives. — 

 It is suggested * that if such an objective as Powell and Lealand's 

 new formula homogeneous-immersion -jVt (aperture = 142° in crown 

 glass of index 1 • 5 nearly, by means of a front lens greater than a 

 hemisphere), were provided with two extra front lenses, one giving 

 an aperture in glass of, say, 115° and one giving 90^, we should be 

 enabled to view objects through a considerable range of thickness of 

 covering-glass, approaching in each case to the maximum aperture 

 that could be used, and hence, probably, we should find much less 

 need of \ or jL objectives. 



By the homogeneous-immersion formula adopted by Powell and 

 Lealand the focal distance is practically a constant quantity ; it 



* ' Eng. Mech.,' xxxii. (1880) p. 84. t See this Joiirnol, ante, p. 8S6. 



