Binocular Microscopes 33 



to notice, cannot linger to reflect upon the sesthetic aspect. 

 In the examination of rotifers and other inhabitants of 

 ' ponds and rock pools,' perhaps the most charming subjects 

 that the microscope has ever revealed, the microscopist 

 with a monocular instrument cannot possibly appreciate 

 and interpret structure and movement in the same accurate 

 manner that the binocular enables him to do. These facts 

 should receive careful consideration when a microscope is to 

 be chosen, but it must be borne in mind that the Wenham 

 stereoscopic form of binocular cannot be used with an 

 objective having a higher air angle than 40°. Provision is, 

 however, always made whereby the prism may be with- 

 drawn, and the light then only goes up the monocular or 

 straight tube, and the instrument is to all intents and 

 purposes as useful and convenient as the monocular micro- 

 scope, while the unemployed eye of the observer is rested 

 by looking into the blank binocular tube ; the fact of its 

 not being illuminated will scarcely be noticeable. 



For use with the binocular microscope, the closer the 

 posterior lens of the objective is brought to the prism the 

 better. In fact, some makers have constructed objectives 

 in very short mounts, working quite on to the prism, up 

 to a power of ^ inch. These, however, are not recom- 

 mended. Dr. Carpenter some time since pointed out that 

 when an objective having an air angle exceeding 40° was 

 employed with the Wenham binocular, spherical objects 

 became distorted, and instead of appearing round in shape 

 they became conical. The special apparent advantage 

 gained is therefore useless. 



Special high-power prisms are also made by two or three 

 opticians, whereby higher powers may be employed with 

 a binocular microscope ; but we doubt whether the results 

 obtainable with them are really worth the additional outlay. 

 In those we have seen there has been such a very unequal 

 illumination in the two tubes, that, personally, we should 

 prefer using the instrument monocularly. It must be 

 understood that all vision through the microscope in the 



3 



