140 



SCIOPTICON MANUAL. 



" The accompanying rude drawing will give you Borne 

 idea of the plan. 



"The front A and the rest B are 

 connected by two iron braces and slide 

 up and down, for the desired focal 

 distance, above the lens C. The ad- 

 justable shelf D supports the object to 

 be copied. The glass plate is laid 

 upon the rest B over the opening, 

 and a black cloth thrown over the top. 

 The instrument is six feet in height, 

 and eighteen inches square, and cost 

 me about two days' labor. It is highly 

 satisfactory." 



It will be understood that the ob- 

 ject of roughing the gelatine, as in- 

 dicated in the above letter, is that it 

 m 31 may present an opaque surface to 



render the image visible, like the focus- 

 ing ground-glass in a photographer's camera; this rough- 

 ing also gives it a " tooth " to hold the markings. 



Ground-glass itself is sometimes used for obtaining 

 drawings by superposition, which is rendered transparent 

 by a coat of varnish, and it might be used in this case ; it 

 is, however, somewhat expensive. A surface is now 

 produced on glass quite similar by what is called the 

 ground-glass varnish. This surface after receiving the 

 drawing may also be rendered transparent by a coat of 

 common varnish, if dried in sufficient heat to prevent 

 its again becoming opaque. 



ASPHALTUM VARNISH. This common black varnish, 

 flowed upon quarter-plate glass and allowed to dry, 

 gives a surface which can easily be drawn upon with a 



