ILLUMINATING APPARATUS 



41 



up on one side, pre- 

 tlie apertures in the 



of light reflected through the object glass itself upon the 

 object. 



This illuminator is generally known as a vertical illuminator, use of 

 It is screwed into the nosepiece of the microscope, and the object m^^itor. 

 glass screwed into the illuminator mount. The body of the 

 microscope is then racked into the 

 position where the object glass is ap- 

 proximately in focus. The illuminator, 

 which can be turned round, should be 

 rotated until the two apertures in the 

 mount are pointing one to each side, 

 while the milled head, which carries Pi«. 32.— No. 3363. 

 the thin glass reflector, is at the front 

 of the instrument. A lamp is now set 

 ferably the left-hand, at the height of 



illuminator, so that light will shine through the two apertures 

 upon a card held on the right-hand side. A bull's-eye con- 

 denser may now be placed in front of the lamp, and the beam 

 of light concentrated by this means. The mOled head, which 

 carries the thin glass reflector, should be turned round until 

 the light is reflected downwards through the object glass 

 upon the object. The nulled head has engraved on it a line 

 parallel with the reflector, and this enables the reflector to be set 

 to approximately the correct angle (45°) before commencing work. 

 The microscope can now be accurately focussed, and a slight 

 alteration of the position of the lamp or the reflector will throw 

 the light in one direction or the other. 



For critical work the lamp-flame or source of illumination 

 should appear, if the object be flat, as a small sharp image on its 

 surface. This can be efiected by having the lamp, if used alone, 

 about 6 inches away from the microscope, or, if a bull's-eye 

 condenser is used, by adjusting the distance of the bull's-eye 

 from the lamp. 



A disadvantage of this form of illumi- 

 nation is that the surfaces of the lenses of 

 an object glass are convex and that » certain 

 amount of light is reflected back into the 

 eye by these surfaces, tending to produce a 

 glare ; but this can frequently be overcome 

 by small adjustments in the position of the 

 lamp or reflector so that such reflections 

 are directed on the sides of the interior of 

 the microscope. 



Another form of this illuminator is made in which the trans- prism 

 parent glass reflector is replaced by a small prism which occupies iUummator. 

 half the aperture of the object glass. This method reflects more 

 Ught, but reduces the aperture and resolution of the object glass. 

 The prism illuminator gives more light when used with low powers 



Fig. 33.— No. 3364, 

 Prism niuininator. 



