POND LIFE 249 



prism system in the eyepiece is not great, and very short 

 exposures can be given with a good light. Photographs 

 taken with this apparatus are reproduced on Plate 38. 



Dark ground illumination was used for Cyclops quadri- 

 cornis, and is particularly useful for such forms as Rotifers 

 and Polyzoa. The preparation of permanent mounts of 

 such delicate organisms as Rotifers is a difficult operation, 

 involving a very careful narcotisation, followed by killing 

 and fixing at the right moment with J% osmic acid or 

 Hermann's solution. They are mounted in 2J% formalin 

 on slides with a central depression. This difficulty in 

 obtaining lifelike permanent mounts is a strong argument 

 in favour of photography of the living organism whenever 

 possible, or when the rapidity of their movements is not 

 too great. Advantage must often be taken of some 

 method to slacken the movements sufficiently ; in some 

 cases, Cyclops for instance, gently warming the slide will 

 effect this. Mr. C. F. Rousselet, F.R.M.S., whose work on 

 Rotifers is so well known, advises cocaine as a narcotic, 

 and the solution given on page 268 may be added drop 

 by drop to a watch-glass of water in which the Rotifers 

 are swimming until their movements are seen to be slow 

 enough to permit of a suitable exposure. 



Photomicrography of Solid Objects The following 



notes apply to a very wide range of subjects ; natural 

 history specimens of all sorts, small objects of virtu, and 

 the like, that can be photographed entire. Some speci- 

 mens may be dry, others preserved in fluid ; some require 

 a microscope for adequate magnification, for others a 

 microscope cannot be employed, and a camera with a 

 short-focus anastigmat lens is used. They are illuminated 

 by sunlight or diffused daylight, or by lamps in conjunc- 

 tion with condensers or reflectors. The arrangement of 

 the illuminant is important, as good contrasts and an 

 appearance of relief and solidity depend on its proper 

 management. Either vertical or horizontal cameras can 



