162 THE MICKOSCOPE. 



mirror may be cut off by a screen, having various-sized 

 apertures placed below the stage. 



The Diaphragm, fig. 98, is the instrument used for 

 effecting this purpose. It consists of two plates of brass, 

 one of which is perforated with four or five holes of dif- 



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 Fig- 98. — The Diaphragm. 



ferent sizes ; this plate is of a circular figure, and is made 

 to revolve upon another plate by a central pin or axis ; 

 this last plate is also provided with a hole as large as the 

 largest in the diaphragm -pi ate, and corresponds in situa- 

 tion to the axis of the compound body. To ascertain 

 when either of the holes in the diaphragm-plate is in the 

 centre, a bent spring is fitted into the second plate, and 

 rubs against the edge of the diaphragm-plate, which is 

 provided with notches. The space between the smallest 

 and largest is great enough to use for the purpose of shut- 

 ting off all the light from the mirror. 



Gillett's Illuminator, or Condenser. — The advan- 

 tages of employing an achromatic condenser were first 

 pointed out by Dujardin, since which time an object-glass 

 has been frequently but inconveniently employed ; and 

 more recently achromatic illuminators have been con- 

 structed by most of our instrument makers. Some years 

 since, Mr. Gillett was led by observation to appreciate the 

 importance of controlling not merely the quantity of light 

 which may be effected by a diaphragm placed anywhere 

 between the source of light and the object, but the angle 

 of aperture of the illuminating pencil, which can be effected 

 only by a diaphragm placed immediately behind the 

 achromatic illuminating combination. Such a diaphragm 

 is represented in fig. 99, manufactured by Mr. Ross : it 

 consists of an achromatic illuminating lens c, which is 



