330 



RECORD OF CURRENT RESEARCHES RELATING TO 



Ahrens' Arrangement for using the Wenham Prism with High 

 Powers. — One difficulty in using tlie Wenham prism with the higher 

 objectives arises, as is known, from the fact that the prism is at some 

 distance from the back lens of the objective, part of the field being 

 thus cut off. Mr. Wenham some years ago constructed some small 

 prisms (one of which was shown at the February meeting), which 

 were attached to a tube fitting into the objective and bringing the 

 prism very close to the back lens : this method, which is otherwise 

 entirely successful, requires each objective to be specially fitted, and 

 an arrangement for turning the prism so that it is in the proper 

 position. 



Mr. Crisp at the same meeting showed (with an |) an arrangement 

 devised by Mr. Ahrens, which enables the prism to be brought close 

 to the back lens with any objective. The tube of the Microscope, 

 instead of terminating in the ordinary way with the universal screw, 

 has two grooves, in which slide two adapters, having the universal 

 screw at one end to receive objectives. One of these carries a 

 Wenham prism, mounted in a projecting tube, so that when the 

 objective is screwed to the adapter the prism dij)S into it, and is thus 

 brought closer to the back lens. 



When it is desired to use the Microscope as a monocular, the 

 adapter containing the prism is slipped off, and the other, which is 

 without a prism, takes its place.* 



Fig. 19. 



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Fig. 20. 



Powell and Lealand's Improved Immersion Condenser. — Messrs. 

 Powell and Lealand have improved the arrangement of the dia- 



* At the Marcli meeting, Dr. Gibbes exhibited a ,L-incli honiogeneoiis-im- 

 meibion objective, with the ordiuary Wenham priam, both fields being fully and 

 equally illuminated. See Proceedings of Meeting of 10th March, infra. 



