240 SOME THOUGHTS ON LIGHT. 



through hundreds of miles of air, but cannot penetrate more than 

 the fraction of an inch into opaque materials. Strictly speaking, 

 it may be said that no substance is either perfectly transparent or 

 perfectly opaque, but practically speaking, some substances are 

 impervious to light-rays. That branch of investigation which is 

 concerned with the refraction of light and similar phenomena, is 

 called Dioptrics. 



The application of the laws of refraction accounts for the 

 numerous deceptive atmospheric effects included in the term 

 mirage, and the investigation of Dioptrics has led to a complete 

 understanding of the mechanism of the eye; the result of which 

 has been the perfection of both the microscope and the telescope. 

 Refraction is a common property of light, but is not equal in 

 different kinds of light. Independently of their difference in the 

 degree of brightness, they differ in the quality of refrangibility and 

 absorption, such difference when distinguished by the eye consti- 

 tuting colour. Differences of colour undistinguished by the eye 

 constitute polarisation. All the different qualities of light termed 

 colour exist in solar rays, and it is both interesting and pleasing to 

 separate the varied rays by means of spectrum analysis. 



If we study the action of light as exhibited in Photography, we 

 shall enjoy hours of amusement and instruction watching the 

 action of such light on a variety of chemical substances. This 

 investigation would well repay any earnest student, and much 

 might be said on this subject, but for the present we must let it 

 pass. We will just say, in conclusion, that the fact that Capt. 

 Abney has proved the possibility of photographing colour,* ought 

 to be sufficient inducement to encourage unremitting attention to 

 this important branch of physical research. 



A picturesque Lake-dwelling has just been discovered under a 

 peat bog near Milan, which differs in many respects from those 

 previously discovered in Switzerland and Upper Italy. The posts 

 are still standing upright, and the planks have been made by 

 roughly splitting trees without the use of any kind of saw. 



* It has not, however, been possible as yet to fix the colours. — Ed. 



