ON PHOTO-MICROGRAPHY. 199 



It is very difficult to lay down any general rule 

 for length of exposure, since there are so many 

 variables thickness and colour of the object, 

 colour of the medium (balsam especially sometimes 

 acquires a yellow colour highly objectionable), focal 

 length and angular aperture of the object glasses, 

 and the number of lenses and kind of glass of which 

 they are composed, length of camera, rapidity of 

 plates, presence or absence of eye-piece, and mode 

 of illumination, all have to be taken into account to 

 give some idea of what may be expected ; we append 

 particulars of a few of our negatives. The first 

 three were taken with an A eye-piece, and a lamp 

 having a 1 inch wick only. The plate was about 12 

 inches from the object. 



" Tongue " of Blowfly. Balsam. Two inch objec- 

 tive by Stanley. Home made plate. Five min. 

 Ferrous oxalate developer. Under-exposed. 



Diatom. Coscinodiscus Excavata. ^ inch objec- 

 tive and achromatic condenser. Home made plate. 

 10 min. Pyro. Correct exposure. 



Diatom. Auliscus sculptus. % inch objective by 

 Beck, and achromatic condenser. Edwards' plate. 

 26 min. Pyro. Rather over-exposed. 



The following were taken without an eye-piece, 

 and with a lamp having a 1^ inch wick. The 

 distance of the plate from the object is given in 

 each case. 



Tongue of Gat injected carmine and stained. Trans- 

 verse Section. Balsam. 3 inch objective of 12 by 

 Browning. Distance 12f inches. Miall's plate. 10 

 sees. Pyro. Correct exposure. 



Rachis of Pteris Aquilina. Transverse Section un- 

 stained. Gly. jelly. 2 inch objective by Stanley. 



