78 ELEMENTARY CHEMICAL MICROSCOPY 



by it through the objective attached below. The rays I, I enter 

 through a circular opening 0. The size of this opening may be 

 changed by turning the collar C which is provided with circular 

 openings of three different diameters. 



Adjustment of Vertical Illuminators. When the object to 

 be examined is small and is supported upon a glass object slide 

 it is always advisable to place below the object slide a piece of 

 black paper, card or other dark opaque object, so that no trans- 

 mitted light can enter the objective. 



The size of the spot of light concentrated upon the preparation 

 should correspond approximately to the area of the preparation 

 made visible in the microscope by the particular objective em- 

 ployed. It is therefore desirable that the diameter of the bundle 

 of rays projected upon the reflecting device shall be adjustable. 

 It is also usually best that these incident rays be nearly parallel. 

 These two requirements are met by interposing between the 

 radiant and the illuminator a suitable lens or series of diaphragms. 

 In the better grades of illuminators, lenses and diaphragms are 

 made an integral part of the apparatus. 1 



The source of incident light should be a powerful radiant, as, 

 for example, a small arc lamp, tungsten or Nernst incandescent, 

 or inverted Welsbach gas burner, acetylene light, or stereopticon 

 lamp with concentration filament, or better still a nitrogen filled 

 tungsten. In all cases the radiant should be as close to the 

 illuminator as is possible for convenience and safety. With 

 powerful radiants and condensing lenses, it is wise to interpose 

 between radiant and illuminator a water cell of moderate thick- 

 ness to act as a cooling device. 



With very highly polished surfaces the image obtained is often 

 of such dazzling brightness as to be almost blinding; in such 

 cases a piece of greenish or blackish glass should always be inter- 

 posed between radiant and illuminator or placed above the eye- 

 piece. 



\ 



1 The 4 to 5 ampere arc lamps for microscopic purposes are generally fitted 

 with a plano-convex condensing lens; in such an event no other lens between 

 radiant and illuminator may be required. The lamp should stand 8 to 12 inches 

 from the illuminator. 



