264 CORRESPONDENCE, 



arranged the mica film, &c., is revolved in its place by turning the 

 cell D, as both cells are geared together with fine racked teeth. F is 

 a polarizing prism which is mounted on an eccentric arm, and can be 

 brought central with the axis of the condenser when required in use, 

 or thrown out as shown in woodcut when not wanted. G is a rack 

 dovetail slide fitting into microscope for the purpose of focussing the 

 condenser on the object. The advantage of this condenser over all 

 other contrivances of this description is that the polarizing prism, 

 selenite films, dark ground and oblique light stops, .are brought close 

 under the optical combination A. The instrument has been devised 

 by Mr. James Swift. 



Mr. ToUes' .^th Objective. — The 'Boston Journal of Chemistry' 

 announces that Mr. Tolles has alone made an object-glass of this power, 

 but that Journal is evidently unfamiliar with the fact that Messrs. 

 Powell and Lealand have long since achieved a similar result. The 

 paper in question says, " Boston stands pre-eminent in the production 

 of exquisite and wonderful optical instruments. Mr. Tolles has just 

 achieved the great result of producing a -^ objective for microscopic 

 uses, a glass of such difficult construction that we believe no optician 

 has ever attempted it before. The power of this objective is such that 

 a single white blood corpuscle covers the entire field of vision. Mr. 

 Tolles has produced two of the finest Jg- objectives ever constructed, 

 one of which is in this city, the other in the hands of a Western 

 gentleman. The angular aperture of one is 120° ; that of the other 

 and the last constructed, is 165°, The objectives are of great excel- 

 lence, and, in the opinion of competent microscopists, far surpass in 

 defining poiver and clearness of field those of European maJce." ! ! 



CORRESPONDENCE; 



Zeiss's Object-glasses. 



To the Editor of the ' Monthly Microscopical Jonrnal.'' 



SiE, — I have received from W. G. Lettsom, Esq., some particulars 

 of the objectives made by Zeiss, of Jena, and I have had, by the kind- 

 ness of the same gentleman, an opportunity of trying one of his small- 

 angled and low-priced |ths. This glass is the D of his catalogue, and 

 the angle of aperture is stated at 60°. I have found this glass so 

 useful, on account of its working distance, penetration, and excellent 

 action with dark-ground illimiination and eye-pieces A, B, and C, of 

 the Ross series, that I am induced to call the attention of English 

 microscopists and opticians to the advisability of not confining their 

 operations to glasses of large angle, as is too usually the case. The 

 small-angled ^th would not compai'e for a moment with a large-angled 

 English |th or ^th in resolving troublesome diatoms ; but for many 



