ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 



103 



threshold " of the eye. The absorbing medium is a liquid of neutral 

 tint, best formed from fine China or Indian ink mixed with water, and 



Pig. 14. 



allowed to stand for 48 hours ; the coarser particles are then deposited, 

 and the supernatant liquid is employed. The apparatus is shown in 

 fig. 14, and, in section, in fig. 15. A variable depth of the liquid is 



