1884-85.] Edinburgh Naturalists' Field Club. 263 



larger end — the smaller end being drawn over the end of the 

 microscope tube and tied with a string. Both the camera and the 

 microscope are attached to the board by means of small brass bolts 

 and screws, and both can be shifted along the board either way by 

 means of a narrow central opening, — the opening for the microscope 

 being on the board itself, and that for the camera in the top of the 

 platform on which it stands. 



The lamp I use is a small microscope lamp, with a half-inch 

 wick. I find this lamp gives sufScient illumination, even for very 

 high powers. For powers up to half-inch, the ordinary bull's-eye 

 condenser in front of the lamp gives quite enough light to enable 

 one to work with short exposures ; and when using higher powers, 

 an achromatic condenser is used to further concentrate the light. 

 I never use the microscope with the eye-piece in. Without the 

 eye-piece I get a sliarper, better-lighted picture, and therefore a 

 quicker exposure. Of course, the magnification in the camera is 

 correspondingly less. With this camera, for instance, fully drawn 

 out, the magnif^'ing power, tested with Smith & Beck's micrometer 

 and Zeiss's quarter-inch objective, is only 125. The image on the 

 sensitive plate is, however, without the eye-piece, much finer and 

 sharper. When you have few object-glasses, it may be necessary 

 sometimes to use an eye-piece to bring up the power to what 

 you want ; but it is much better to do without tlie eye-piece, if 

 possible. 



A great deal has been said and written about the non- coinci- 

 dence of the chemical or actinic and the visual foci of microscopic 

 object-glasses. I do not find in practice that I experience any 

 difficulty on that ground. My glasses were all chosen for their 

 good qualities as microscopic objectives simply, and with no view 

 whatever to their use for photography. I have never required 

 with any of them to apply any correction for the actinic focus. 

 I simply get as sharp a focus on the camera screen as I can with 

 powerful magnifiers, and I can be absolutely certain that I shall 

 get a correspondingly sharp photographic negative. This will 

 not apply, however, to object-glasses with only one combination. 

 My experience is, that glasses of that description do require correc- 

 tion for the chemical focus. 



I use an ordinary piece of ground-glass to focus the image of 

 the object in the camera in the first instance, and to get the light 

 nicely in the centre of the screen ; but this is far too coarse for 

 the final adjustment, and when finally adjusting the focus I put 

 in a screen formed of a piece of plain glass, one side of which 

 has been daubed slightly over with glazier's putty, and then spread 

 over the glass with long strokes by the forefinger ; or a similar 

 piece of glass which has been rubbed over slightly with virgin-wax, 

 — then the glass is slightly lieated to melt the wax, and rubbed in 



