36 HOW TO USE THE MICROSCOPE 



the reflector, and light rays collected by the reflector 

 are fooussed by it on to the object. Or the light 

 is further strengthened by centring the bull's-eye 

 in line with lamp and reflector, and throwing parallel 

 rays on to the latter. 



It is useful to practise the parallelization of light 

 by means of the bull's-eye. This result is obtained 

 when an image of the light-flame is formed on the 

 wall of the room about 10 feet distant. This image 

 secured, note distance of lens from the flame, and 

 at that distance an approximately parallel beam of 

 light will always be obtained. A parallel beam 

 from the bull's-eye thrown on to the mirror below the 

 stage will often be of great value in illuminating 

 transparent objects, and it is also usefvil, thrown 

 directly on to the object, in the practice of photomi- 

 crography. 



The student will find it necessary to take care of 

 his eyes, and he will be enabled to do so by toning 

 the field with tinted glass, as already mentioned 

 (p. 32), and also by accustoming himself to use 

 either eye with ease. In using the microscope, the 

 eye should always be placed close to the ocular, but 

 it must not be pressed upon it. 



The owner of a good microscope, howsoever 

 generously he be disposed, is well advised not to 

 allow any but expert persons to use his instrument. 



