VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 79 



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 radi- 

 ant 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 



When dealing, however, with a vertical illuminator of simple 

 type having no parallelizing or condensing lens, excellent results 

 may be obtained by using an objective as a condenser and thus 

 projecting a very bright beam upon the prism or disk. Objec- 

 tives may also be employed for illuminating objects with oblique 

 rays. The author employs 32 and 48 mm. photographic objec- 

 tives with iris diaphragms for this purpose. 



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- 



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 same is true of the newer low volt- 

 age, concentrated filament, " Mazda " lamps. The lamp should stand 8 to 12 

 inches from the illuminator. 



