294 APPENDIX. 



hard, sections are cut as with the ordinary instrument 

 except that the razor Used must be dry, not wetted with 

 spirit. The sections are placed in weak spirit (1 part 

 spirit 2 parts water) if a hardened tissue, in salt solution 

 if fresh, to dissolve out the gum, and they can then be 

 mounted in any desired manner. 1 



Gum is useful for the freezing process because it ac- 

 quires a cheesy consistence when frozen, in place of be- 

 coming hard and crystalline (Urban Prit chard), 



Microphotographic Apparatus. Photography is 

 every day coming more into use for obtaining images of 

 microscopic objects. The mode of application consists 

 in the adjustment of a photographic camera to the tube 

 of the microscope (which it is better to divest of its eye- 

 piece); the image formed by the objective is received and 

 focussed upon the ground-glass plate of the camera, the 

 objective of the microscope representing in fact the lens 

 of the ordinary camera. Either wet or dry sensitized 

 plates are employed, and the mode of exposing the plate 

 and all the subsequent processes of developing, fixing, 

 and printing are in every respect the same as in ordinary 

 photography. Jt will be sufficient, therefore, in this place 

 to describe a convenient form of apparatus (Fig. 40), and 

 the manner of obtaining an image of the microscopic ob- 

 ject ; an account of the subsequent procedure, and the 

 precautions it is necessary to take, will be found in recog- 

 nized standard works on photography. 



Since it may often be required to photograph wet pre- 

 parations, the microscope should be kept vertical, and the 

 camera must therefor be supported above the microscope. 

 This is done by means of a strong mahogany frame con- 

 sisting of a stand, .?, upon which the microscope is placed 

 and four vertical pillars rigidly connected below to the 

 stand, and above to one another by cross-pieces. Two 

 brass rods, r, one on either side, run vertically between 

 the stand below and the cross-piece, and serve more imme- 

 diately to support the camera, r, which is capable of slid- 

 ing up and down on the rods and can be fixed by screw- 

 clamps at any height. 



The camera should be lightly constructed, with a bel- 



1 For a more complete description sec Rutherford, 

 Practical Histology." 2d edition, L87G. 



