II 



MICROSCOPE AND ACCESSORIES. 



Fig. 17 .—Double Nose-Piece 



position. In removing the ocular, reverse the operation. If the above 

 precautions are not taken, and the oculars fit snugly, there is danger in 

 inserting them of forcing the body of the micro- 

 scope downward and the objective upon the object. 



§ 32. Putting an Object under the Micro- 

 scope. — This is so placing an object under the 

 simple microscope, or on the stage of the com- 

 pound microscope, that it will be in the field of | 

 view when the microscope is in focus (§ 33). ^ 



With low powers, it is not difficult to get an ob' 

 je'ct under the microscope. The difficulty increases, 

 however, with the power of the microscope and the 

 smalluess of the object. It is usually necessary to 

 move the object in various directions while look- 

 ing into the microscope, in order to get it into the 

 field. Time is usually saved by getting the object 

 in the center of the field with a low objective be- 

 fore putting the high objective in position. This 

 is greatly facilitated by using a double nose-piece, 

 or revolver.* 



§ 33. Field or Field of View of a Micro- or Rcvolver for Quickly 

 scope. — -The area visible through a microscope changing Two objectives. 

 when it is in focus. When properly lighted, and there is no object un- 

 der the microscope, the field appears as a circle of light. When exam- 

 ining an object it appears within the light circle, and by moving the ob- 

 ject, if it is of sufficient size, different parts are brought successively 

 into the field of view. 



In general, the greater the magnification of the entire microscope, 

 whether the magnification is produced mainly by the objective, the 

 ocular, or by increasing the tube-length, or by a combination of all 

 three (see Ch. Ill, under magnification), the smaller is the field. 



The size of the field is also dependent, in part, without regard to 

 magnification, upon the size of the opening in the ocular diaphragm. 



* As specimens are sometimes very small, or some part of a large specimen 

 shows a particular structure with special excellence, it is desirable to mark the 

 preparation so that the minute object or the part of a large object may be found 

 quickly and with certainty. A simple way to do this is to find the object under 

 the microscope, and then place a minute spot of black ink at one side. After this 

 is done, remove the slide from the stage and surround the object with a ring of 

 shellac cement, making the ring as small as possible and not cover the object. It 

 will then always be known that the part to be examined is within the ring (B. 1, 

 47, C. 117). The enclosure in a ring may also be very elegantly done by the use 

 of a marking apparatus like that of WinkePs (B. K. & S., p. 48), making use of 

 either a diamond point or a delicate brush dipped in shellac or other cement. 



