3G0 Mr. C. Tomlinson on the Motion of 



deposit ; that artificial light has no effect in producing a deposit ; 

 that the deposits are sometimes furthest from the sun, and at 

 other times nearest : these are termed aphelion and perihelion 

 movements. Dr. Draper says, " The sun's rays have the 

 power of causing vapours to pass to the perihelion side of vessels 

 in which they are confined ; but, as it would appear, not at all 

 seasons of the year. For example, I have a certain glass fitted 

 up for making these observations; and in this vessel, during 

 December, January, and part of February 1836-37, a deposit 

 was uniformly made towards the sun ; during March, April, and 

 May next following, although every part of the arrangement 

 remained to all appearance the same, yet the camphor was depo- 

 sited on the side furthest from the sun. It does not appear 

 that any immediate cause can be assigned for this wayward- 

 ness/' 



It was further found that when the sun's light was passed 

 through water and solutions of ammonio-sulphate of copper 

 and of bichromate of potash, the crystallization was on the aphe- 

 lion side. 



The following statement is also made : — " Light which has 

 suffered reflexion at certain angles seems to have undergone a 

 remarkable modification, being no longer able to put the glass 

 into such a condition that it can cause motion towards the sun. 

 Under such circumstances crystallization proceeds with rapidity, 

 not on the perihelion side of the vessel, but on the opposite side. 

 This result is not supposed to be due to polarization, as it takes 

 place at all angles." 



A very few words wdil explain Dr. Draper's theoretical views. 

 From the results of an experiment made with a differential ther- 

 mometer, he l( cannot admit that the rays of heat have any active 

 part in bringing about the phenomena." He was strongly inclined 

 to the opinion, from the action of the tinfoil rings, that the 

 motion of camphor to the light was an electrical phenomenon ; 

 but he failed to detect electricity, though sought for by means 

 of a delicate electrometer. His ultimate conclusion seems to 

 have been that these deposits are due to a mechanical action of 

 light. In short, the result of Dr. Draper's elaborate inquiry 

 was to multiply phenomena and to leave the theory as it was. 



I am not aware that this subject has been investigated by any 

 one since the publication of Dr. Draper's volume. In repeating 

 his experiments, I found it necessary to get rid of the air-pump 

 and of exhausted vessels altogether, in order to be at liberty to 

 multiply experiments to any extent. In some comparative trials 

 it was found that raw camphor was more sensitive than refined, 

 and it was accordingly adopted. Common corked and stoppled 

 bottles of white and coloured glass were used, varying in capa- 



