METHODS FOR FOCUSSING. 



211 



its best visual, while a 2-inch, by Dancer, required one 

 complete revolution of the milled head of the fine adjust- 

 ment. 



Ordinary ground glass is too coarse to focus upon ; two 

 surfaces should be rubbed together with a little of the 

 finest emery and water, or a focussing glass may be 

 prepared by coating an ordinary glass plate with the follow- 

 ing varnish : 



Gum mastic 40 grains 



Gum sandarac 1 60 grains 



Ether 4 oz. 



Benzol i^ oz. 



J 



The correct focus may be obtained by the eye alone, 

 though this is difficult, especially with high powers; and, 

 therefore, it is advisable to em- 

 ploy a magnifier of some de- 

 scription an engraver's lens or 

 an ordinary eye-piece with the 

 eye-glass removed as shown in 

 Fig. 194. The field lens is 

 placed next the eye and the 

 open brass tube slid in a pa- 

 rallel direction over the glass 

 focussing screen. 



The author now uses a much 

 improved method for focussing : 

 removing the ground glass 

 slide another is substituted of 



FIG. 194. 



mahogany, but pierced with a series of seven holes, into 

 each of which the ordinary A eye-piece may be fixed. 

 The thickness of the slide is such that when the eye- 

 piece is pushed in as far as it will go the diaphragm 

 lies in the same plane as the ground surface of the 



P 2 



