334 SUMMARY OF CUKRENT RESEAECIIES RELATING TO 



pronounced if the oil-immersion objectives employed are of medium 

 power. 



With certain objectives of comparatively low power and small 

 working distance (e.g., a 6-mm. Holos of the earlier construction, N.A. 

 of * 84) it has been found possible to convert a dry objective into an 

 oil-immersion. 



C4) Photomicrography. 



Practical Principles of Plain Photomicrography.* — Under the 

 above title George West has written a book which will be found to be 

 a very useful and comprehensive manual. He dedicates the volume to 

 E. M. Nelson, and in the foreword states that it is not intended for 

 competition with any of the large and important works dealing witli 

 photomicrography already before the public, but rather to supplement 

 such publications on a few points and to give the writer's personal 

 experiences. Its greatest novelty is perhaps the recommendation of the 

 system which was practised by Dr. J. J. Woodward in the United 

 States fifty years ago, and has somehow been overlooked by subsequent 

 operators. Woodward used sunlight as an illuminant, reflecting it by 

 means of an adjustable mirror through a tiny aperture in a shutter into 

 an absolutely dark room. In this room in the path of the sunbeam 

 stood the Microscope, and behind it an adjustable easel for the wet 

 plate, no camera being employed. The disadvantages of depending on 

 sunlight in this country are too obvious, and so West modifies the 

 method by using artificial hght, placing the latter along with the 

 Microscope in a light-proof chamber, through a hole in the wall of 

 which the tube of the instrument projects. The plate is placed on an 

 easel in another chamber, to wit the dark-room, where the exposure is 

 made and the plate developed. The photographic apparatus for this 

 purpose is of the simplest form ; no camera or dark-slide is used, and 

 the local carpenter, or anyone handy with tools, can readily construct 

 all that is required at a small cost. This plan obviates the difficulty 

 often experienced with the ordinary camera, especially when a number 

 of different objectives are in constant use, of getting the centre of 

 the projected image (which must be central with the optical combina- 

 tion) to coincide with the centre of the screen, as it is merely a matter 

 of arranging the latter to suit the projected image. Another point is 

 that the extension, and, consequently, the magnification, is only restricted 

 by the length of the room. It is, therefore, as easy to expose a 20 by 

 20 in. plate as a 3 by 3 in., should one require a large picture for 

 demonstration purposes. Another great advantage gained by this 

 simple apparatus is the very important one of being able to see the 

 image in the dark-room on a dull white card placed on the easel in 

 front of the screen. This among other conveniences enables one to 

 detect and rectify any fault, such as unevenness in the illumination, in 

 a way that is impossible with the ordinary camera. The method also 

 offers other advantages such as screening off with a card the effects of 

 sudden vibration and regulating the exposure of parts requiring varying 



* Campbell, Sons and Co., Dundee (1916) xand 146 pp. (8 pis. and many figs.). 



