TRANSACTIONS OF THE SECTIONS. 45 



mind tlie consideration of the path of rays iu a medium of continuously variable 

 index of refraction ; that he then thought it easiest to calculate the path of the 

 ray by translating the problem into the emission theorj^, and treating the ray as a 

 moving body acted on by a force depending on the variation of the index of refrac- 

 tion, and so proceeding by an artifice justifiable on the ground that the emission 

 and undulation theories are mutually equivalent in respect to the course of rays 

 ■when the proper alterations of the hypotheses are made ; and that my brother 

 showed him, on the other hand, how easy it is to begiu with the right hypothesis 

 bji- making the velocity inversely proportional to fi, and calculating the change of 

 wave-front. 



Professor Maxwell, in 1853, sent to the ' Cambridge and Dublin Mathematical 

 Journal ' a problem about the path of a ray in a medium in which 



''-^+?' 



where jUf, and a are constant, and r is the distance from a fixed point. Such rays, 

 he points out, move in circles. This problem, he mentions, was intended to illus- 

 trate the fact that the principal focal length of the crystalline lens of the eye is 

 very much shorter than anatomists calculate it, from the curvature of its surface and 

 the index of refraction of its substance. The reason, he shows, is the increase of 

 density towards the centre of the lens, so that the rays pass nearly tangentially 

 through a place where the density is varying. Also, in the Cambridge Exauiina- 

 tious for 1870, Prof Maxwell set a question about the conditions of a horizoutal 

 ray of light having a greater curvature than that of the earth. A great deal, Jie 

 says, has been written about atmospheric refraction by Bessel, Clairaut, and others ; 

 and a question has been set on it iu January of every year at Cambridge for several 

 years back, so that the subject has been much discussed in various ways; but, he 

 says, the mode of treatment of the subject in the present paper does not seem to 

 have been anticipated. — J. Thomson. 



On a Phenomenon connected loith Diffraction. By T. Ogiek AVaed, M.D. Oxon. 



The author has observed that when he stands at sunset on a hill at such a dis- 

 tance from another hill that his shadow reaches its vanishing-point before arriving 

 at it, instead of a shadow there is diffused light, due to diftractiou, more or less in 

 extent in proportion to the distance, and that this light does not disappear until 

 the observer has descended 22° into the shadow of the hill. He throws out the 

 supposition that the bright sky 22° round the sun has a similar power to produce 

 dift'raction, and asks whether the sun's corona can be merely this diffracted light, 

 and suggests that dming the progress of an annular eclipse the unshadovred portions 

 of the earth ought to receive an extra portion of light from the diffi-acted light sur- 

 rounding the shadow of the moon. 



On tJie Imjyortance of the Salts of Uranium in PhotograjfJiij. 

 By Colonel Stuart Wortley. 



The great advantage of obtaining photographic negatives by means of a sensitive 

 emulsion in lieu of using the collodion and bath separately is beginning to be gene- 

 rally recognized by those who take au interest in the advance of scientific photo- 

 graphy. The advantages obtained by this method of working are, first, that the 

 condition of one substance alone, viz. the sensitive emulsion, has to be considered ; 

 and, secondly, that a greater degree of sensitiveness can be obtained than by the 

 bath process. 



In order to obtain this exalted degi'ee of sensitiveness with an emulsion it is ne- 

 cessary, after the formation of a certain amount of bromide of silver, to saturate the 

 emulsion with as much free nitrate of silver as it will hold in solution. This prin- 

 ciple has been recognized by all the most advanced workers since the author first drew 

 attention to such conditions being required iu a paper read before the London 



1872. 5 



