LIGHT. 117 



as to render it unannealed, in which state its molecules are in 

 a state of tension or strain and the glass highly brittle, and 

 on replacing it in the polariscope, a beautiful series of colours 

 is perceptible. Instead of subjecting the glass to heat and 

 sudden cooling, let it be bent or strained by mechanical pres- 

 sure, and the colours will be equally visible, modified accord- 

 ing to the direction of the flexure, and indicating by their 

 course the curves where the molecular state has been changed 

 by pressure. So I have found that if tough glue be elongated 

 and allowed to cool in a stretched state, it doubly refracts 

 light, and the colours are shown as in the instance of glass. 



Submit a series of crystals to the same examination, and 

 different figures will be formed by different crystals, bearing 

 a constant and definite relation to the structure of the par- 

 ticular crystal examined, and to the direction in which, with 

 reference to crystalline form, the ray crosses the crystal. 



In the crystallised salts of paratartaric acid, M. Pasteur 

 noticed two sets of crystals which were hemihedral in opposite 

 directions, i.e. the crystals of one set were to those of the 

 other as to their own image reflected in a mirror ; on making 

 a separate solution of each of these classes of crystals, he found 

 that the solution of the one class rotated the plane of polarisa- 

 tion to the right, while that of the other class rotated it to the 

 left, and that a mixture in proper proportions of the two solu- 

 tions produced no deviation in the plane of polarisation. Yet 

 all these three solutions are what is termed isomeric, that is, 

 have as far as can be discovered the same chemical constitution. 



In the above, and in innumerable other cases, it is seen that 

 an alteration in the structure of a transparent substance alters 

 the character and effects of the transmitted light. The pheno- 

 mena of photography prove that light alters the structure of 

 matter submitted to it ; with regard even to vision itself, the 

 persistence of images on the retina of the eye would seem to 

 show that its structure is changed by the impact of light, the 

 luminous impressions being as it were branded on the retina, 

 and the memory of the vision being the scar of such brand. 

 The science of photography has reference mainly to solid sub- 

 stances, yet there are many instances of liquid and gaseous 

 bodies being changed by the action of light : thus hydrocyanic 



