of the atmosphere , &c. 377 
The planets also differ much from each other in this re- 
spect. The moon, Venus, and Jupiter, seem to possess every 
colour ; but the green is very pale in all of them. Mercury 
and Mars appear deficient in the middle and most refrangi- 
ble rays, whilst the light of Saturn seems to be composed 
principally of the mean rays with a very small proportion of 
the extreme colours of the prism.* 
The different refrangibility of the differently coloured rays 
is very visible in stars near the horizon. If viewed on a fine 
night with a power of 200 and upwards, they appear expanded 
into a prismatic spectrum. Sirius, when within a few degrees 
of the horizon, presents a most beautiful object. 
Having remarked the very oblong figure which the spec- 
trum assumes when near the horizon, and found from repeated 
observations of different stars that the separation of light 
begins to be visible as high as 40° or 50° of altitude, I was 
led to believe that the dispersive power of the atmosphere 
must be sufficient, in many cases, to produce considerable 
effect on astronomical observations ; and, consequently, to 
suppose that it would be desirable to ascertain, if possible, the 
exact degree of separation of the several rays.-f 
With this view, therefore, I began a series of observations ; 
* .Query. May not this circumstance explain why Saturn, though less brilliant, 
bears magnifying better than Jupiter and Venus? 
f Dr. Herschel, in a note to his Paper on Double Stars, published in the 
seventy-fifth volume of the Philosophical Transactions, says that the prismatic power 
of the atmosphere is very visible in low stars ; and very justly observes that this power 
ought not to be overlooked in delicate and low observations : he gives the measure of 
two diameters of t Sagiftarii, which seem to indicate that the refraction of the extreme 
rays is about -~f ±, the mean refraction. I think it due to that great astronomer to 
mention the circumstance, though it was totally unknown to me till long after I had 
completed my observations on Mars. 
sc 
MDCCCXV. 
