488 JAMES CLERK MAXWELL. 



of this work, of these water-colour drawings he sent some 

 to Nicol, who presented him in return with a pair of Mcol 

 prisms of his own construction, which are now, with the 

 original colour -top, polariscope, specimens of unannealed 

 glass, etc., in the Cavendish Laboratory. His experiments 

 on the passage of light through solids exposed to strain 

 suggested to Maxwell the employment of polarised light as 

 an analyser of the strains in the different portions of an 

 elastic solid when exposed to mechanical stress, and led to 

 the production of the paper on " Elastic Solids " read before 

 the Eoyal Society of Edinburgh in February 1850; but 

 before giving an account of this paper we must mention 

 another investigation connected with the physiology of 

 vision. 



There are some persons who, when they look at a point 

 in the sky distant 90 from the sun, at once observe two 

 conspicuous yellow brushes with their axis in a plane passing 

 through the sun, while the space between exhibits the com- 

 plementary violet colour. This phenomenon was first 

 noticed by Haidinger in 1844, and is known as Haidinger's 

 Brushes. The appearance is only transitory, and disappears 

 in a very short time if the eye be kept directed to the same 

 point in the heavens. The same appearance is produced 

 whenever " plane-polarised " light enters the eye ; as, for ex- 

 ample, when light is reflected from a polished surface of glass 

 at a particular angle. But there are some persons, on the other 

 hand, who are apparently incapable of seeing Haidinger's 

 Brushes, or only see them with difficulty. Generally it is 

 persons of a dark complexion who see Haidinger's Brushes 

 more readily than others, and in such persons the yellow spot 

 appears to be peculiarly insensitive to blue light. A paper 

 on this subject was read before the mathematical and physical 

 section of the British Association by Professor Maxwell in 

 1866 (Report, Part II.) He found that on looking through 



represented natural objects. A kettle-holder which used to hang by 

 the fireside was a representation of a square of unannealed glass when 

 placed between two crossed Nicol prisms. 



