ACCESSORY INSTRUMENTS. 



125 



Fig, 74. 



Messrs. Powell and Lealand supply, with some of their 

 microscopes, a diaphragm of the form represented by fig. 74, 



when used, it is 

 adapted to the 

 stand of the large 

 condensing lens, 

 and placed in front 

 of the lamp, at 

 about eight inches 

 distant from the 

 mirror ; it consists 

 of two plates of 

 thin sheet iron, 

 blackened ; one of 

 these is of a circu- 

 lar figure, being provided with five holes of different sizes, and 

 capable of being revolved upon a larger plate in the same way 

 as the diaphragm before described, as adapted to the under 

 side of the stage. When this diaphragm is used, an image of 

 the size of the aperture employed, should be shown on the 

 mirror ; by this, only a part of the field of view will be illumi- 

 nated, the centre will be light, but around the margin there 



will be darkness ; this oftentimes is 

 very useftd in rendering very deli- 

 cate markings more distinct. The 

 size of the illuminated spot will de- 

 pend upon that of the aperture 

 employed, and also upon the relative 

 distances of the mirror from the ob- 

 ject, and of the diaphragm from the 

 mirror. 



Erector. — Those microscopes fur- 

 nished with a draw-tube are capable 

 of having adapted to them the erector 

 or erecting eye-piece ; this is repre- 

 sented by fig. 75, as being screwed 

 into the lower end of the draw-tube ; 

 it consists of a piece of brass tube. 



Fiff. 75. 



