NEGATIVE MAKING 77 



scopes. It may be interesting to some if I 

 briefly note the procedures in making the 

 original negative: Objective, a first-class & 

 of moderate aperture, by Beck's; ocular, 

 Spencer's aplanatic, i-inch; Ross' achromatic 

 substage condenser; plate, medium landscape; 

 screen, cobalt blue; radiant, acetylene; ex- 

 posure, 2% minutes ; developer, eiko-hydro. 



Opaque Illumination by Reflected Light. As 

 already noted, the great majority of micro- 

 scopical subjects are viewed by transmitted 

 light, being naturally transparent or rendered 

 so by artificial means ; but a vast number are 

 opaque ; the surfaces only can be seen as illu- 

 minated by light reflected therefrom. Usu- 

 ally these require very low or medium powers 

 for their rendition ; in fact the illumination of 

 an opaque object under high amplification is 

 impossible, excepting with certain classes of 

 subjects and by means of special illuminating 

 devices. In metallography it is a most im- 

 portant adjunct in studying the characters of 

 many metals and alloys, their polished surfaces 

 being examined under magnifications from 100 

 diameters to more than 1,000. In high power 

 opaque work the objective is made to be the 

 illuminator as well, either by means of a prism 

 or a disc of thin glass placed within its mount. 



