ILLUMINATION GO 



yo that the luminous .source is circular, and no more than 

 the required size (Conrady, Beck) ; second, the condenser 

 may have a fairly long focus (8.5 to 12 millimeters), so 

 that the image of the source is large enough to fill the 

 field of view of the medium and high powers ; third, a prism 

 may be used instead of a mirror, so that there is only one 

 image. (For the low powers, a disc of ground glass can be 

 turned in, in front of the flame.) A series of thin metal 

 diaphragms may be arranged to fit close to the flame 

 (or ground glass). For this method, the condenser is 

 corrected for a lamp distance of about 25 centimeters. 

 It may be convenient to fix lamp and microscope on a 



Fig. 17. — Diagram to show how glare may be produced. By the water- 

 immersion condenser, of which the upper lens only is shown, an image of a 

 suitably sized source of light can be thrown on the object, so as just to light up 

 the object-field; that is, that part of the object visible through the microscope. 

 This is shown by the black central spot. Then no extraneous light enters the 

 objective, and there is, consequently, no glare. With a source of light, however, 

 which gives an image in the plane of the object larger than the circle seen through 

 the eyepiece, extraneous light enters the objective, and there is glare. This 

 condition is shown by the dotted part of the slide. 



board, since the centering of the two is important, though 

 some latitude is allowable. 



If electric light must be used, as is usual in laboratories, 

 no ordinary electric lamp has a continuous incandescent 

 surface large enough to fill the field of low and medium 

 powers, or even often of the high powers. This might 

 be done perhaps by incandescent gases, such as mercury 

 vapor, or neon; but such lamps are apparently not manu- 

 factured in a convenient form for continuous microscope 

 work. 



Use of Ground or Matt Glass. — An eflficient substitute 

 for the flame, which can be used with the electric lamp, 

 is a matt surface of glass to refract and diffract the light, 

 and thus act as an original source. This is, of course, 



