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Transactions of the Societij. 



It is, therefore, most important to keep the water in ohservation 

 or photographic aquaria at a constant temperature, and is best 

 accomplished by inlet and outlet syphons, so that a gentle circu- 

 lation is always taking place. By this means the creatures are 

 kept in a healthy condition, and their movements and appearance 

 normal. The photomicrographs of living Hydroids have been 

 oljtained in this way, using a zoophyte trough mounted on a special 

 stage, and with a low-power objective attached to the Reflex camera. 

 When a higher magnification is necessary, I use one of Messrs. 

 Charles I>aker's E.M.S. Microscopes, which I consider one of the 

 most perfect instruments for biological research, and in conjuction 

 I either use the Iveflex camera, or a Leitz demonstration eye- 

 piece and the ordinary photomicrographic camera. 



Fig. 15. — Stereoscopic Photomicrograph of Young Echinus, taken with the 

 Stereo-binocular Microscope and Camera as shown in figs. 13 and 14. 



In studying such forms of marine life as the Polyzoa, true 

 stereoscopic photomicrographs are of very great assistance, and 

 by the method I employ are comparatively easily obtained. I 

 used Baker's model of the Greenough Stereo-binocular Microscope. 

 This is placed on a baseboard, which has an upright to carry the 

 stereoscopic camera — a 7x13 c.cm. Verascope of fixed focus. The 

 object on the stage is focused through the eye-pieces of the Micro- 

 scope, and the camera is then gently lowered into position, so 

 that the lens almost touches the front lenses of the Microscope 

 eye-pieces, and the plate is exposed (fig. 13). As the lenses 

 of the camera are set at infinity, there is no need for further 

 focusing, if the eyes of the observer are normal and he has in 

 the first instance obtained a critically sharp image through the 



