34 



suspected — nothing less in fact, than that Jupiter shines 

 partly by his own light," etc. Guillemin asks :--" Does the 

 globe of Jupiter still possess an internal, heat considerable 

 enough to raise the temperature of the crust to an extent 

 sufficient to make up for the relative feebleness of the solar 

 heat ?" (and light, he might have said) " These are questions 

 on which science is still silent." Herschel, Ball, and Webbe, 

 pass the question by in silence. Mr. Proctor, however, argues 

 strongly in favour of the giant planets being more or less 

 self-luminous. 



The recent conjunction of Jupiter and Mars (28 June) 

 was looked forward to with interest by me, as furnishing a 

 favourable opportunity for photometric experiments with refer- 

 ence to the relative intrinsic brightness of these two planets ; 

 and I commenced my preparations several days before hand. 

 At the risk of being somewhat tedious, I think it will be well 

 to describe in detail the means I adopted, in order that the 

 results may be judged of at just what they are worth, and 

 no more. 



I constructed a diaphragm (or shutter), for the telescope 

 (8|in. reflector), consisting of two half-discs, pivoted at their 

 circular centres, and graduated on their rims in degrees, 

 thus : — 



