50 PRACTICAL PHOTOMICROGRAPHY. 



piece, the operator being in a comfortable and convenient 

 position, sitting or standing, as desired. These things being 

 done, the platform is turned back to a " stop " which is so 

 arranged by the maker of the apparatus that the optical sys- 

 tem is then axially centered with the photographic system. 

 The focusing on the plane of the sensitive plate alone remains 

 to be performed, and this is done by means of the rod seen in 

 the cut. The rod has a pulley, the fine adjustment has a screw 

 with a grooved milled head, and a pulley passing over the rod- 

 pulley and round the grooved milled head operates the fine ad- 

 justment at the will of the operator examining the image on 

 the focusing screen of the camera. A " Hooke's J oint " may 

 be used in place of this focusing arrangement, the writer used 

 that contrivance for some time, but on the whole an arrange- 

 ment with rigid rod and pulleys is to be preferred. 



Figure 16 shows the writer's latest arrangement, which 

 combines some ideas gathered from Mr. Nelson's apparatus 

 with others of the apparatus last figured, and still others which 

 occurred one by one to the writer as he advanced in experi- 

 ence. The platform carrying light and optical system and 

 turning on its pivot is retained, so also is the rigid rod and 

 pulley contrivance. The platform is still " stopped " at a 

 certain point, but this time the camera is entirely free on 

 a very heavy teak base to which also the swinging platform 

 is attached. In Fig. 15, after the optical part is stopped at 

 the axis, the front only of the camera is run forward to meet 

 the ocular end of the microscope tube ; but in Fig. 16 the whole 

 camera is pushed forward to the cap on the tube, and in both 

 cases a smaller cap fixed to the camera fits very loosely inside 

 a larger cap on the microscope tube. There is no difficulty in 

 centering the latter arrangement, Fig. 16, for the centre of 

 the camera focusing-screen is marked, and if the centre of 

 the object coincides with the centre of the ground glass 

 everything must be centered and perpendicular to the general 

 axis. 



Any photographic camera will answer for this work pro- 

 vided it be light-tight, and reasonably well made. No " mo- 

 tions," such as "swing-backs," are required. The camera 



