4^4 Herschel/s Observations and Remarks on the 
planets is visible. To distinguish the figure of Jupiter pro- 
perly it may be called an ellipsoid, and that of Saturn a 
spheroid. 
Observations on the periodical Changes of the Colour of the 
polar Regions of Saturn. 
In the observations I have given on the planet Mars, it has 
been shown that an alternate periodical change takes place in 
the extent and brightness of the north and south polar spots ;* 
and I have there suggested an idea that the cause of the 
brightness might be a vivid reflection of light from frozen 
regions, and that the reduction of the spots might be ascribed 
to their being exposed to the sun. 
The following observations, I believe, will either lead us to 
similar conclusions with respect to the appearance of the 
polar regions of Saturn, or will at least draw the attention of 
future observers to a farther investigation of the subject. 
With high magnifying powers the objects we observe 
require more light than when the power is lower ; this affords 
us a good method of determining the relative brightness of 
the different parts of a planet. The less bright object will be 
found deficient in illumination when the power exceeds what 
it will bear with ease. I have availed myself of this assistance 
in the observations that follow. 
June 25, 1781. With an aperture of 6 , 3 inches I used a 
magnifying power of 460. This gave a kind of yellowish 
colour to the planet Saturn, while the ring still retained its 
full white illumination. 
November 11, 1793. From the quintuple belt towards the 
* Phil. Trans, for 1784, page 260. 
