On a new Photo-Lithographic Process. 177 



into which I shall not enter. In using glass negatives on 

 stone, we encounter another, and an almost insurmonntahle 

 obstacle, when the stone exceeds a very moderate size; 

 namely, the difficulty of ensuring intimate and perfect con- 

 tact between all parts of the two rigid surfaces. A sensitive 

 surface exposed in this way receives the ink upon its sunned* 

 or altered parts. 



Thirdly, Exposure under a Positive. 



Some of the disadvantages of the previous method of 

 producing the photographic image which is to take the ink, 

 are obviated by having recourse to exposure under a positive ; 

 others again of a very serious nature are superadded. The 

 rigidity of the glass is avoided if we use a paper positive, but 

 one of this kind is not nearly so positive in reference to 

 transmitted light as we would suppose, when looking at it 

 with reflected light; that is, the contrast is not so great 

 between the quantity of light which passes through the 

 black and the white parts of such a print, and is greatly 

 inferior to the contrast in the quantity of light which passes 

 through the transparent and opaque parts of a good intense 

 negative. Seal positives can be prepared upon glass, coated 

 with albumen of a very intense character, but the manipula- 

 tions are troublesome, and would not be adopted save in cases 

 of necessity. Exposure under a positive is tantamount to 

 exposure in the camera, and the surface so treated must take 

 the ink upon the unsunned parts. 



I shall now enumerate the several photo-lithographic pro- 

 cesses of which I have been able to gain any information. 

 M. Lamercier, in 1853, and Mr. Macpherson, in 1856, have 

 published processes almost in every respect identical ; they 

 both assure us of very successful results, as far as drawings 

 showing a " grain" are concerned, that is in a style suitable 

 for landscapes and portraits, but indifferently adapted for 

 maps, plans, music, &c, &c. The " sensitive surface" they 

 use is formed on the stone by running a sort of varnish, made 

 of asphaltum, over it, and the exposure is accomplished 

 under a common negative. The circumstances which have 

 prevented their methods coming into general use are, no 

 doubt, chiefly the following : — 



1st. Great irregularity and uncertainty in the photo-chemi- 

 cal constitution of each different sample of asphaltum. 



2nd. The expense caused by the extravagant use of ether, 

 large baths of which are required, in which to dip and soak 

 the face of the stones. 



