170 SELECTION AND TJSB 



The light is also sometimes modified by passing it through 

 ground or colored glass blue being a special favorite. Such 

 light-modifiers, as they are called, produce a pleasant and 

 equable illumination, which is a great relief to the eyes, but, 

 except for the resolution of finely lined objects, we have not 

 found them otherwise of any special advantage. When it is 

 desired to obtain the greatest resolving power that a lens is 

 capable of affording, the blue cell, as it is called, is probably the 

 most efficient accessory. This is simply a glass tank, somewhat 

 like a zoophyte trough, filled with a solution of oxide of copper 

 in liquor ammonise. The solution is prepared by adding 

 liquor ammoniae to a saturated solution of sulphate of cop- 

 per, until the precipitate which is first formed is re-dis- 

 solved. The intensity of the blue may be regulated, either 

 by diluting the solution, or by varying the thickness of the 

 layer of liquid. 



When it is desired to examine anything by light reflected from 

 it, instead of light transmitted through it, the object should be 

 placed before a dead-black surface, such as the dark part of the 

 diaphragm, or a blackened card, and at such a distance from it 

 that the surface of the background is not in focus. Then, 

 place the condensing lens in relation to the lamp, so that a 

 bright spot of light will fall on the object, and on bringing it 

 into focus it will be clearly seen. Low powers only can be 

 satisfactorily used for the examination of opaque objects by 

 beginners. 



The beginner should commence with the simplest mounted 

 objects, and afterwards, when a little skill in the manipulation 

 of the instrument has been acquired, he should proceed to the 

 examination of such simple unmounted objects as are easily 

 prepared. The latter course will prove altogether the most 

 valuable and instructive, for he who confines himself to the ex- 

 amination of mounted objects only can never hope to become a 

 microscopist. After a time, when a little skill has been 

 acquired in the preparation of objects, the student may proceed 

 to preserve and mount them. Most young people try to -mount 

 before they have learned to prepare objects, and the conse- 

 quence is that they soon find themselves in possession of a 

 large collection of very poor slides. 



