THE MICROSCOPE AND MICROSCOPIC METHODS 55 



blood spreads laterally in the angle between the two slides. 

 The second slide is then pushed along the surface of the first with 

 the blood following it in the angle. The thickness of film may 

 be regulated by varying the size of angle between the two slides. 



For staining blood films, either Leishman's or Giemsa's stain 

 or some modification of them should be used as a general rule. 

 After fixation in absolute alcohol, blood films may be stained 

 with Loffler's methylene-blue or by Gram's method. 



Staining Bacteria in Tissues. Pieces of organs about i cm. 

 in thickness may be taken. Alcohol is the best agent for pre- 

 serving them. The hardening will be completed in a few days. 

 It is best to change the alcohol. The amount of the alcohol must 

 be twenty times the bulk of the tissue to be preserved. 



Ten parts of the standard 40 per cent solution of formalde- 

 hyde, with 90 parts water make a good mixture for fixation; after 

 twenty-four hours change to alcohol. 



Imbedding in Collodion or Celloidin. From alcohol the 

 pieces of tissue are placed in equal parts of alcohol and ether 

 twenty-four hours; thin collodion (ij per cent), twenty-four 

 hours; thick collodion of a syrupy consistency (6 per cent) twenty- 

 four hours. The specimen is laid upon a block of wood and sur- 

 rounded by thick collodion, and then inverted in 70 per cent al- 

 cohol. The collodion makes a firm mass, surrounding and perme- 

 ating the tissue, and permits very thin sections to be cut. The 

 soluble cotton sold by dealers in photographer's supplies serves 

 as well as the expensive preparation known as celloidin. To 

 make collodion, dissolve it in equal parts of alcohol and ether. 

 Soluble cotton is also called pyroxylin, and is a kind of gun-cotton. 



Imbedding in Paraffin. (a) Pieces of tissue 2 to 3 mm. 

 thick which have already been fixed in alcohol or formaldehyde 

 are to be placed in absolute alcohol for twenty-four hours. 



(b) In pure xylol one to three hours. 



(c) In a saturated solution of paraffin in xylol one to three 

 hours. 



(d) In melted paraffin having a melting-point of 50 C., 



