CONTRIBUTIONS TO SCIENCE. 487 



When they reach the analyser only that component of each 

 set of vibrations can pass through which takes place in the 

 plane corresponding to transmission through the analyser. 

 Then it may happen that the two rays of light of one par- 

 ticular colour are so vibrating that the waves interfere and 

 destroy one another, so that this colour is completely absent 

 in the transmitted light while other colours are partially 

 destroyed. The result is that the combination of colours 

 which passes through produces a particular tint, and this 

 will be different according to the obliquity of the ray, the 

 thickness of the glass, and the amount of strain it has ex- 

 perienced, for on the latter depends the difference of velocity 

 of the two rays, and consequently the particular colour de- 

 stroyed in the case of glass of a given thickness. 



Maxwell's first serious experiments on light appear to 

 have had their origin in his visit to Mr. Wm. Nicol in 

 April 1847. After this visit he constructed a polari- 

 scope of cardboard, employing blackened glass mirrors as 

 polariser and analyser. This was supplied with lenses 

 for use when a conical beam of polarised light was required. 

 The lenses were mounted in cardboard frames. A very 

 similar instrument constructed by him shortly afterwards, 

 but of wood instead of cardboard, together with the lenses 

 mounted as before on cards, is still preserved in the 

 Cavendish Laboratory. Maxwell, after returning from 

 Mr. Mcol's, prepared some samples of unannealed glass 

 by heating pieces of thick plate glass to redness and 

 allowing them to cool rapidly. By means of a camera 

 lucida adapted to the cardboard polariscope he observed 

 and faithfully copied in water-colours some of the chromatic 

 effects exhibited by these plates of glass, showing the 

 manner in which the glass was strained by the rapid cooling. 

 Some of these figures are shown in Plate III., and will 

 be again referred to. 1 As mentioned in the previous part 



1 Maxwell had a great faculty for designing, and would frequently 

 amuse himself by making curious patterns for wool-work. His designs 

 are remarkable for the harmony of the colouring. Sometimes they 



