186 MICROSCOPY. 



proper appliances, such, for instance, as the serai-cylinder origin- 

 ally introduced for this purpose by Mr. Tolles. 



A slit one-half or one-third as wide as the field gives equally 

 good results where water-contact of the objective is used, but when 

 working through air reduces the angle to an evidently fallacious 

 degree. Glycerine is a better substitute for glass than water is, 

 and therefore for a thin cover and an objective corrected for best 

 work through thick covers, glycerine should always be preferred 

 to water as the immersion-fluid. 



" 180°" Angular Aperture. — The latest contribution to this 

 subject is a note from Mr. Wenham, which, besides its personal- 

 ities, consists of a violent attack upon an article in the Naturalist 

 for advocating " 180°." As the article referred to simply mentioned 

 the 180° as the claim of a certain optician, and did not say one 

 word in favor of the propriety of the claim, the minor inaccura- 

 cies of Mr. Wenham's note may well pass uncorrected, while its 

 sneering tone will be rated at its true worth by those who are 

 familiar with the courtesies of literature and science. The treat- 

 ment which Mr. Wenham has received from the Naturalist for 

 years past is sufficiently well known. 



Lest any future writer, rather than use a tiresome circumlocu- 

 tion, should unfortunately say 180° when he wished to say indefi- 

 nitely near to 180°, it would be well to have it understood that no 

 one need consider it either necessary or handsome to make such 

 unguarded expression the occasion of an abusive reply. 



Caps for Mounting Opaque Objects. — Prof. John Peirce of 

 Providence, K. I., has had a die cut for making a novel kind of 

 cell, which is excellent for mounting large opaque objects, such as 

 many mineral specimens and nearly all seeds, which require to be 

 permanently preserved and at the same time show best without a 

 cover-glass. The cell is made of thin copper, and has the shape 

 of a hat with a very low crown. The rim at the bottom is to be 

 cemented with marine glue to the glass slide, and the top of the 

 crown is a removable cap slipped on or off at pleasure, so that the 

 object can be examined or manipulated with the greatest facility. 

 Though not prepared for the trade, they could probably be ob- 

 tained, in exchange or otherwise, by any microscopist. 



Rogers' Micrometers and Test Plates. — Mr. Rogers of the 

 Cambridge Observatory has made arrangements to furnish micros- 



