PHYSICAL SCIENCE. 45 



and complete theory of the spectrum, or learnt the many 

 conditions under which light penetrates, pervades, or even 

 becomes latent in different forms of matter, this deficiency 

 is due, not to any want of zeal in research, but to the in- 

 trinsic difficulties of dealing with this subtle and multiform 

 element. Looking at the colours alone of the spectrum 

 so beautifully defined to the eye by the several refrangi- 

 bilities which evolve them controversies still exist as to 

 the manner in which they combine with each other ; and 

 until the recent suggestion of Sir J. Herschel, followed by 

 the very valuable researches of Professor Maxwell (1861), 

 which rescue green from its subordinate place as a compound 

 of yellow and blue, we were at fault even as to what might be 

 deemed the primary colours in the scale. The connections of 

 another portion of the spectrum, with those changes in the 

 molecular conditions of matter which we call chemical, furnish 

 a new avenue to the higher physical laws which lie before us. 

 In photography, the exponent of these changes, we have an 

 agent so working as almost to put to shame the manipula- 

 tions of human art. To fix what we transiently see by reflec- 

 tion from a mirror, was the object sought for, and for a long 

 time ineffectually. Once attained, every year has added to 

 the beauty and perfection of an art, which may especially be 

 called the child of science, and which we must not yet affirm 

 to have reached its maturity. Whoever gazes on the colours 

 of the solar spectrum, cannot but be seduced into thinking 

 that photographic chemistry may hereafter afford colouring 

 to our pictures, as well as the simple imagery of light and 

 shade. We are still too imperfectly acquainted with the 

 complex machinery of the solar beam to warrant abandon- 

 ment of the object ; even were it not probable that the 

 phenomena of colour are mainly due to the atomical con- 

 stitution of the bodies recipient of light, and to the organic 

 structure of the eye itself, forming the last material link with 



