130 PEACTICAL PHOTO-MICKOGKAPHY 



embodies certain ideas founded on mechanical principles that 

 are not very generally appreciated, some description of the 

 arrangement may be of interest. 



The apparatus was originally designed by Dr. T. A. B. 

 Carver and the writer for use at the Lister Institute of Preven- 

 tive Medicine, and the objects aimed at were to combine 

 perfect rigidity with ease of manipulation. The supporting 

 portion of the apparatus consists of two light iron girders which 

 are very strongly braced together (Fig. 47). On the top of 

 these, two steel rods are supported, and the camera slides to 

 and fro along them. Any part of the camera is adjustable 

 along the rods, and can be very firmly clamped in any position. 

 The bellows are made in segments, so that a very short camera 

 or one of a length up to six feet may be used as required. The 

 microscope, illuminant, and other optical parts are supported 

 on a tail-piece which may be swung out on a central pivot, 

 allowing of convenient access to the microscope (Fig. 47 b). 

 The end of this tail -piece nearest to the camera is supported 

 on -a semicircular arm, which projects out from the side of the 

 cast-iron frame, and which serves as a support for the front 

 end of the tail-piece when it is swung out of alignment with the 

 camera. The illuminant, collecting-lenses, and the microscope 

 are carried on a triangular metal bar, as in the Zeiss apparatus 

 already described, and each unit of the apparatus is on a 

 saddle-piece, which allows of adjustment in a vertical direc- 

 tion and in the line of collimation, and may be clamped in any 

 position. 



There are two methods of focussing which may be adopted 

 alternatively. In one a long steel rod runs along the entire 

 length of the base of the instrument between the girders, and 

 is provided with movable brass milled heads at intervals, so 

 that focussing can be effected from any position of the focussing- 

 screen of the camera. At the end of this rod, immediately 

 under the pivot which supports the swinging- out tail-piece 

 carrying the microscope and illuminant, is a grooved pulley. 

 Over this pulley a waxed silk cord is carried, and uniform tension 

 of the cord is maintained by means of a weight at each end. 

 This cord passes through a central hole in the pivot supporting 

 the swinging-out tail-piece, and is led up to a double-grooved 

 pulley which is on the table supporting the microscope 3 and 



