ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 805 



the impossibility of getting the image focused in a satisfactory 

 manner, the great rapidity of the dry gelatine plates now in use 

 making the time of exposure quite a secondary matter. 



The arrangement for making the photographs is as follows : — In 

 front of the condenser of the lime-light lantern is fixed a tube 10 in. 

 in length, at the further end of which is placed a plano-convex lens, 

 of about 2 in. focal length, mounted in a sliding tube movable by 

 rack and pinion, the beam of light passing through which comes to a 

 focus, and then while only slightly divergent falls on the achromatic 

 condenser fixed in the substage of the Microscope. This arrange- 

 ment gets rid of most of the heat-rays ; the beam passing through the 

 condenser traverses the object to be photographed, the image of which 

 is projected directly on the screen by the object glass, no eye-piece 

 being used. For focusing, a sheet of glazed white paper is used 

 pasted on a glass plate placed in the dark slide. By focusing in 

 this manner as one sits in front of the screen the various adjustments 

 of the Microscope and condensers are easily made, while keeping a 

 distinct view of the image. 



As regards the details of focusing the image, he adopts the 

 following method : — 



The object having been brought into the desired position and 

 roughly focused, it is then by means of the mechanical stage re- 

 moved from the field, and the diaphragm aperture which is intended 

 to be used in the particular case having been placed in position, the 

 achromatic condenser and light are manipulated until the field is 

 evenly illuminated ; the diaphragm plate is then revolved until the 

 full opening is reached ; the object is then brought back into position, 

 and the best possible image obtained by means of the fine adjustment ; 

 the diaphragm plate is then again returned to its former position ; the 

 image, of course, gains much in sharpness, and although quite 

 sufficiently bright to produce an impression on a rapid plate, is not 

 at all in as satisfactory a condition for accurate focusing as when 

 presenting a brighter appearance. 



"When all the adjustments have been made, the sleeve suspended 

 from the frame is placed in position, one end of it being attached to 

 the sliding front of the camera, and the other end to a pasteboard 

 cylinder, which fits on to the back of a narrow box, containing a 

 sliding shutter by which the exposure is made. To the front of this 

 box the body of the Microscope is attached by a small black velvet 

 sleeve which completes the camera. The large sleeve is made of 

 mackintosh cloth, with three hoops fastened inside to prevent its 

 collapsing. 



Mitchell's Focusing Glass for Photo-Micrography- *— G. O. 

 Mitchell, finding that no matter how finely tho focusing screen was 

 ground, it would not allow the finer details of objects to bo seen, 

 made use of a Iluyghonian eye-piece in the following manner. A 

 narrow strip of thin board, 15 X 2 X 3/8 in., had a circular hole cut in 

 its ccntro through which the eye-piece could be just forced with con- 



* Aiinr. Mon. Mi.T. .Jouru., v, (1884) p. 81 (1 fig.). 



