KEVIEWS. 
419 
Hence this is devoted less to what may he called commercial applications of 
the art, and more to the theoretical aspect of the subject. 
It commences with a historical sketch of the early experiments of 
Wedgwood, Niepce, Daguerre, and Talbot, occupying five pages, followed 
by four pages of experiments on Light. These hear traces of haste, due 
probably to the necessity for great condensation. For instance, silver chloride 
is described as lima cornwa, light as a fluid, and parallel rays in the dark 
chamber are made, by the illustration on page 7, to pass with unaltered 
parallelism through a condensing lens and a prism, and still uninverted, to 
give a real image on a screen. 
With Chapter III., containing the theory of sensitive compounds, we 
reach more congenial ground, and in the 36 chapters which follow, it is 
remarkable how much practical information has been condensed into a little 
over three hundred pages. Indeed it is not possible with limited space to do 
more than enumerate the chief topics introduced. Sensitive silver, iron, and 
chromium salts ; asphaltum, dyes, chlorine and hydrogen, sulphur and anti- 
moniuretted hydrogen as influenced by light, form the bulk of the fourth ; the 
daguerreotype, collodion, the wet process, development, intensifying and 
fixing, each furnishing successive chapters. An account of wet-plate manipu- 
lation, and its defects, both in negative and positive pictures, is followed by 
dry plate, gum, gallic, and albumen-beer processes. Emulsion processes, the 
gelatino-bromide and calotvpe processes, are each separately described. 
Silver printing and its manipulation is adverted to at some length; as is 
printing with iron, uranium and chromium salts. Photo-lithography, photo- 
engraving and photo-collotype bring to an end the list of methods. The 
twenty-ninth section describes the lens, and the thirtieth the camera, tents, 
dark rooms, and other apparatus It may be noted here that perhaps the 
laws of refraction, dispersion, aberration, conjugate foci, and some other 
common places of optics, in a case where space is evidently precious, might 
well have been left to rudimentary text-books on that subj ect. Ch apter thirty- 
one on the picture, the choice of a point of view, grouping, and composition, is 
of remarkable value, illustrated as it is by some charming little woodcuts. A 
stranded boat by Manners Gordon, and a view on the Thames by Woodbury, 
positively light up their respective pages. There is a return, in the three 
following chapters, on actinometry, photo-spectroscopy, and thermal photo- 
graphy to purely theoretical considerations ; and in those on solar, stellar, and 
celestial work we have a good resume of the astronomical value of this fasci- 
nating art. Micro-photography, and some miscellaneous applications, such as 
that to meteorological registration, to military service, and to the identifica- 
tion of criminals, conclude the work. 
In another edition, the feeble woodcut of temporary photo-spectrum 
apparatus at page 265, the description and drawings of Mr. Browning’s 
automatic arrangement, and of Stoney’s Heliostat, might well be omitted in 
favour of more specifically photographic apparatus. But there is much 
valuable material in the book. 
W. H. Stone. 
B H 2 
