448 MR. THOMAS HEELIS ON THE ZODIACAL LIGHT. 



The great difficulty in such cases as this is, that the ob- 

 servations, even of the same man, at different epochs in a 

 long series of observations are not comparable inter se, 

 especially if the observer have in the course of his observa- 

 tions adopted a theory which insensibly biases him. I in- 

 cline also to think that the light changes its form more 

 than can always be fairly accounted for by differences in 

 our atmosphere ; but although I have devoted much time 

 and thought to the subject, both in observing and con- 

 sulting the observations of others, I am completely at a 

 loss to account for the phenomena observed, upon any 

 theory hitherto broached ; nor do I believe that a sufficient 

 number of reliable facts have been collected to allow of 

 any one undertaking the task of forming a theory with 

 any hope of success. The question has been mooted 

 whether the variations observed have any connexion with 

 the solar- spot period; but at present I do not think that 

 this question can be solved, for want of observations. The 

 daring way in which the American observations assign the 

 boundaries of the light, even when it is involved in the 

 Milky Way, and the evident bias which leads their author 

 to observe phenomena which, if true, would bear out his 

 peculiar view, but which have never been observed by any 

 one else although carefully looked for, renders this mass 

 of observations (for there are upwards of 350 charts) very 

 doubtful. I am aware of the numerous chances of error 

 incident to this class of observations, and that my own are 

 by no means free from errors ; but I can conscientiously 

 say that they are free from bias. 



