SUNDEY APPARATUS 



75 



Fig. 87.— Focus- 

 sing Glass. 



a focussing glass should be used ; this consists of a high-power roouaaing 

 lens mounted in an adjustable tube, which can be set so that^'*^' 

 when it is stood upon the ground glass the latter is sharply 

 focussed. A small portion of the ground glass screen in the centre 

 is left clear so that the image can be 

 viewed with the focussing glass without 

 being partially obscured by the ground glass. 



The method of setting the focussing glass 

 is as follows : Loosen the top cell which 

 hold s the lens combination by slightly un- 

 screwing it, then screw the outer one of 

 the tubes downwards away from the ceU, 

 leaving the screw of the top ceU exposed. 

 Now hold the inner tube and screw the top cell backwards and 

 forwards until a pencil mark on the lower side of the ground glass 

 is sharply defined, while the focussing glass is held against the 

 upper side. Then screw up the outer tube, which wiU form a 

 lock nut and fix the top cell in the correct position. 



The horizontal pattern of photomicrographic camera is Horizontal 

 illustrated in Fig. 88. When this is used, the microscope must ^cto- 

 be placed with its tube in a horizontal position. It enables a^^_ 

 variation in the size of the picture to be obtained according to 

 the extension of the bellows. It is of unusually solid construction. 

 It has an extension of 30 inches and takes a 4J X 3J- inches 

 (quarter-plate) size negative. It consists of a heavy steel 

 hexagonal bar fixed to two steel cross bars which are supported 

 on four levelling screws. Along this bar slide three frames with 

 connecting bellows, each frame being provided with a clamp 

 screw. The frame at one end holds the ground glass or double 

 plate-holder, the frame at the other end carries a flexible bag to 

 attach to the microscope. It can be adjusted up and down so 

 that its centre is at any distance from the ground between 

 5f and 7J inches, and it can be raised to a higher level for use 



Fig. 88. — No. 3340, Horizontal Photomicrographic Camera. 



with large microscopes by putting four feet under the levelling 

 screws. 



There are but few purposes for which a larger photomicroscope 

 than 4^ X 3^ inches isjequired, and for this size 30 inches is 

 ample extension. 



