RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 
3 
stood that M. Daguerre’s pictures admitted of no such reproduction, thus giving a 
decided superiority to the use of paper, or other similar material, provided it could 
by any means be wrought up to equal perfection of effect. 
8. To avoid much circumlocution, it may be allowed me to employ the terms po- 
sitive and negative, to express respectively, pictures in which the lights and shades are 
as in nature, or as in the original model, and in which they are the opposite, i. e. 
light representing shade, and shade light. The terms direct and reversed will also 
be used to express pictures in which objects appear (as regards right and left) as they 
do in nature, or in the original, and the contrary. Thus also we may speak of a po- 
sitive direct, a negative direct, or of a positive reversed or negative reversed picture, 
to the great saving of words and time. Thus the camera impresses on white paper a 
negative-reversed photograph. And this being also the character of an engraving 
transferred on similar white paper, it is evident that a single transfer from a camera 
picture is equivalent in ultimate effect to a double transfer from an original drawing 
or from ah engraving on such paper. 
9. The chief difficulty in the reproduction of drawings or engravings, consists in 
the necessity of operating a double transfer, so as to produce a resulting impression 
of the same character as the original in both these respects, whereas from what is 
above said it appears that this does not apply in the case of a camera picture. Grant- 
ing therefore that such a picture could be impressed with such depth and sharpness 
as to be equal in these respects to a finished engraving, the problem of photographic 
publication is solved, since it is a fact that the negative reversed photographic im- 
pression of an engraving, however highly finished, such as is obtained by a single 
transfer on white paper, is quite equal in precision and finish to the original en- 
graving. 
10. The above remarks relate only to what may be called the practical part of the 
photographic art. We come now to points of scientific interest. It is noticed then 
in the paper above alluded to in abstract, 4thly, That the intensity of chemical ac- 
tion of different rays in the solar spectrum appears to be in great measure discon- 
nected with their colorific impressions on the eye ; and this conclusion, generally 
though unequivocally indicated by a variety of experiments in which coloured glasses 
were used to analyse and separate the incident rays, will be found confirmed, with 
many details equally novel and unexpected, in the experiments about to be related, 
in which the prismatic analysis of the rays has been resorted to. 
11. 5thly. A curious property of glass plates in exalting the effect of sunshine on 
nitrated paper is pointed out; a property of which subsequent experiments have shown 
the reverse to obtain in certain circumstances, and which the results of these experi- 
ments (to be described in the sequel), in place of elucidating, have only served to 
render more enigmatical. 
12. Having no better ground of arrangement under which to group my subse- 
quent experiments, I shall follow this order, so far as it goes, in describing their 
b 2 
