ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 607 



" The camera I use is a whole-plate one, with a long bellows, with, 

 for long-distance work, a telescopic attachment in front (made of 

 rolled brown paper). The upright position of the plate in the camera 

 is the more convenient for photographing opaque objects requiring to 

 be lighted from the front and side, and the objective should be divided 

 horizontally: This also applies to the oblique light method. 



"The reversal of the image on the plate — so that the print will 

 not require to be divided, and simplifying the mounting — may be 

 effected by using a carrier in the dark slide (the whole-plate slide allows 

 this to be done) in which the plate (5 in. by 4 in.) can be placed 2£ in. 

 out of the centre, so as to receive the image from the right-hand (or 

 upper) half of the lens on the left-hand (or lower) half of the plate. 



"Where a diatom does not present its best aspect through being 

 turned the wrong way, the images may be reversed, so as to get a 

 pseudoscopic effect, which renders the object as though photographed 

 from the other side by turning it inside out. 



" The opening in the carrier should be 7J in. by 4 in., a piece of glass 

 2J in. by 4 in. filling the otherwise unoccupied end. A screen (of 

 blackened card or thick paper) with an aperture in the centre of 2| in. 

 by 3 in. should be placed in the carrier to protect one-half of the plate 

 while the other half is being exposed. After exposing one side of the 

 plate, the slide is taken into the dark room, and the plate moved to the 

 other end of the carrier. Then the screen on the objective is moved 

 half round, or the eccentric diaphragm under the condenser is given a 

 semi-revolution (or, in the case of a lens in the draw-tube, the tube is 

 given a semi-revolution without disturbing the focus), and the second 

 exposure can be made. 



"An important thing to remember when photographing opaque 

 objects is that, to secure even illumination of the two halves, the 

 illuminant must be on the same level as the centre of the objective — 

 this is why the horizontal division of the objective and the upright 

 position of the plate arc recommended. 



" For the lighting of large transparent objects, when using the lowest 

 powers, 1 have had a cell, which carries a 4|-in., 6-in., or 8-in. focus 

 uncorrected condenser, fitted to the large aperture under the main 

 stage of the Microscope, and brought as near as possible to the object. 

 For the smaller objects I use an achromatic condenser of 1*0 N.A. 

 This can be altered in power by removing the top combination, or, if 

 necessary, using only the lowest of the three lenses. As illuminant, I 

 use acetylene (the finest light for all ordinary work), from a special 

 burner I had made, limelight, and sometimes sunlight (parallel rays) 

 through a heat-absorbing medium. On the platform carrying the 

 apparatus there is a scale from to 49 in., with the zero in a line with 

 the Microscope stage. This, wjth the aid of tables for the various lenses 

 used, enables me to work to definite magnitudes. 



" Exposure, of course, depends upon colour or brightness of object, 

 illumination, and magnification, and may vary from a few seconds to an 

 hour. 



" I use ordinary developers — such as would give a hard result to 

 accentuate feeble contrasts. .Orthochromatic plates, with or without a 



