DIAPHRAGMS. 



It is made of many degrees of excellence, and therefore 

 every cheap instrument purchased should be specially 

 examined with a low power (3 or 4-inch objective), to see 



FIG. 37. 



whether the apertures are concentric with the axis of the 

 instrument. Every part of the diaphragm should be well 

 blackened, in order to prevent the presence of reflected 

 light, which would interfere with the illumination. 



Collin's graduating diaphragm, illustrated by Fig. 38, 

 was the first of its kind. It consists of four movable 

 shutters, acted upon by a lever, so that the whole of them 



FIG. 39. 

 ^r 



FIG. 38. 



may be moved inwards or outwards simultaneously. The 

 opening thus produced is not circular, but this is of little 

 importance in actual working. 



The graduating or " iris " diaphragm of Messrs. Beck is 

 illustrated in Fig. 39. It differs from that of Mr. Collins 

 only in the number of the shutters, which thereby produce 

 a nearly circular aperture. Mr. George Wale, of New 



