THE MICROSCOPE AND MICROSCOPIC METHODS 55 



for twelve to twenty-four hours. It is again rinsed and now 

 differentiated by immersion in the iron solution until black clouds 

 cease to be given off. When the desired differentiation has 

 been obtained the preparation is washed, dehydrated by passing 

 through graded alcohols, and absolute alcohol, cleared in xylol 

 and mounted in balsam. 



Preparation and Staining of Blood Films.— Blood films are 

 best made on clean, flamed slides. A small drop of fresh blood is 

 received on the surface of one slide near one end. The end of 

 another slide is applied to the first at an acute angle so that the 

 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 as 

 well as by the speed of movement. 



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 b6 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 preserv- 

 ing 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 CoUodion or Celloidin.^From absolute alcohol 

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

 twenty-four hours; thin collodion {i\'^ 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 

 surrounded by thick collodion, and then inverted in 70 per cent 

 alcohol. The collodion makes a firm mass, surrounding and per- 

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



