CHAPTER VIII. 
PHOTOGRAPHIC PLATES. EXPOSURE. 
T N view of the observations lately made and noticed 
1 on page 55, it may be stated at the outset that, if 
ordinary achromatic lenses are to be used, it may be 
taken for granted that orthochromatic plates will always 
be used. Plates “corrected for colour” are made in 
this country by Messrs. Cadett and Neall, of Ashtead, 
Messrs. B. J. Edwards and Co., and the Britannia 
Works Co. at Ilford. The two last firms sell the plates 
under the name of “ Isochromatic.” Briefly put, the 
result of the treatment of the plates or emulsion is that 
the plates are abnormally sensitive to the yellow region 
of the spectrum as compared with the violet and blue. 
Ordinary plates are very much more affected by the 
violet end of the spectrum than by the yellow region ; 
orthochromatic plates are relatively much more sensitive 
to the yellow. When a “ screen ” is used, usually a 
piece of yellow glass, some of the violet and blue rays 
are cut off, and so the yellow-sensitiveness of the plate 
is “ eked out ” by the reduction of the violet and 
blue rays. 
The Isochromatic plates are sold in at least two 
qualities as regards sensitiveness, the “ instantaneous” 
