140 BACTBBIOLOGY. 



microscope, with the left eye, the image of the object will 

 appear on the paper, and can be readily outlined. The 

 relative size of different objects, seen under the microscope, 

 can thus be faithfully reproduced. With a little care and. 

 patience, very satisfactory results will be obtained in a 

 short time. 



Various optical contrivances have been devised to facil- 

 itate the drawing of microscopic objects. The camera lu- 

 cida of Abbe is especially useful for this purpose. 



Cover-Glasses. 



The slides and cover-glasses, employed for microscopic 

 work, must be rigidly clean. The usual method of clean- 

 ing cover-glasses, is to immerse them in alcohol, and then, 

 to wipe with a clean cloth till dry. Frequently, cover- 

 glasses thus treated, although apparently clean, will not 

 permit the spreading of a drop of water on their surface. 

 This thin layer of fatty matter often cannot be removed, 

 even by treatment with sulphuric, or with acetic acid. The 

 cover-glass may be passed rapidly, five or six times, from 

 above downward, through a Bunsen flame, in which case 

 the heat destroys the organic matter on the cover-glass. 

 When thus treated, a drop of water, placed upon that sur- 

 face of the cover-glass, which was in direct contact with 

 the flame, can be spread evenly over the entire surface. It 

 will not gather into small droplets, as is usually the case. 

 The objection to this method is, that many of the cover- 

 glasses crack during heating. 



The following method of preparing cover-glasses will 

 give perfect satisfaction. The cover-glasses are immersed 

 in alcohol, and are then wiped dry, and as clean as possi- 

 ble. They are then placed in an Esmarch or Petri dish, 

 and heated in a dry-heat sterilizer at 170-180°, or higher, 

 for one hour The organic matter is thus subjected to de- 



