ILLUMINATING APPAKATUS 



39 



centrated and brilliant small patch of light which is made use 

 of for the illumination of opaque objects viewed with low 

 powers. 



Fig. 28 shows the Beck electric lamp used with a bull's- oondenser 

 eye condenser in this manner for condensing a powerful beam "^ '"°''' 

 of light upon the 

 top of an object on 

 the stage of the 

 microscope (see also 

 p. 46). 



A method of 

 displaying the struc- 

 ture of opaque ob- 

 jects is sometimes 

 adopted in which 

 two lamps and two 

 bull's-eye condensers 

 are used, one of 

 which has a blue 

 and the other a red 

 glass to colour the 

 light. If these are 

 used at different 

 angles, difficult 

 structure may be 

 more easily inter- 

 preted by observing 

 the different colours 

 of the shadows. 



A suitable illumination of opaque objects is required f or niumination 

 botanical, entomological, and general work, and is of paramount objeot? with 

 importance for metallurgy. When low-power object glasses are ^.Jg^g^ 

 used there is sufficient working distance (see Fig. 3) between 

 the front of the object glass and the object to throw light in from 

 one side by means of a bull's-eye condenser either attached to a 

 lamp, as shown in Fig. 28, or on a separate stand (Fig. 26). 

 When a bull's-eye is used on a separate stand there is another 

 method of using it which is useful for even high powers. If the 



bull's-eye condenser is placed 



with its flat surface upwards, 



nearly parallel with the direc- 



^^..^. tion of the light, as in Fig. 29,' 



• '^'■■^^^ *^® ^S^* enters the curved 



''y^^ surface and is condensed ; when 



it meets the flat surface it is 

 reflected back in such a way 

 that a very powerful narrow ribbon of light is emitted ; this 

 band is so narrow that it can be made to illuminate the object 



Fig. 28.— No. 3332. 



Fig. 29. 



