61 MAMI'I'LATIOX OF A I' I' A HATU8. 



Do not take the objective apart for cleaning. Should any 

 repairs need to be made on the inside of it, return to the maker for 

 examination. For removing balsam or glycerin jelly, see directions 

 elsewhere, (p 56.) Having secured a clear field, i. e., the round 

 area seen on looking into the microscope, some studies should be 

 made in focusing. For this purpose prepare a glass slide as 

 follows: place in the center of a glass slip a piece of paper about 

 1 m. m. square on which is the letter (a) in " diamond " type. Place 

 on this a drop of balsam and cover with a piece of glass about 1 

 c. m. square and 2 m. m. thick. On this piece of thick glass place 

 the letter (b) very close to the letter (a), but not over it ; add a drop 

 of balsam and superimpose a piece of thick glass, the size of the 

 one beneath. Repeat the operation with the letter (c) and place 

 over this a thin cover-glass, the size of the thick glass below. 

 After the balsam has hardened the mount can be sealed with shellac 

 or asphalt. Place the slide under the microscope, using a or 

 1 inch objective. Focus on the letter (b), then turn the fine adjust- 

 ment either way, and observe the effect. By continuing the experi- 

 ment, the working of the adjustments of the instrument will soon 

 be understood. A knowledge of this is absolutely necessary, since 

 with higher powers, the direction in which the micrometer screw 

 should be turned is of the utmost importance. It should also be 

 known whether a certain part seen under the microscope is at the 

 top or bottom of the object. It is only by much practice in this 

 way, that the relations of structures are determined. Observe, also, 

 that only one letter is in focus at once. This is because the depth 

 of focus is not great. In general, this distance become less as the 

 magnification is increased. 



In order to examine thoroughly an object of much thickness, 

 which is readily penetrated by the light, it must be studied in 

 section; that is, focusing on one part and afterward on one above 

 or below it. 



The location of the parts with regard to their vertical 

 position can be determined by the working of the micrometer 

 screw. Such sections of the object as are in focus at any one time 

 are known as "optical sections." Objects with irregular contour 

 must be studied in this way. 



