<_. . - 
On the dispersive Power of the Aimosphere, Sc. 37 
only take the trouble of comparing them on a fine night. They 
present a striking variety of colour even to the nakedeye. But 
this difference becomes still more perceptible when ‘they are 
viewed through a prism properly adapted to the eye-piece of a 
reflecting telescope. 
A star viewed in this manner is converted into a prismatic 
spectrum. Sirius and the lridliant white stars exhibit a large 
brush of beautiful violet, and the most refrangible colours in 
' great abundance. Aldebaran, « Orionis, and the red stars show 
only a small proportion of those colours, whilst the dull white 
stars exhibit a great quantity of intense green light. 
The planets also differ much from each other in. this respect. 
The moon, Venus, and Jupiter, scem to possess every colour ; 
but the green is very pale in all of them. Mercury aud Mars 
appear deficient in the middle and most refrangible rays, whilst 
the light ef 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 spectrum 
assumes when near the horizon, and pond from repeated obser- 
vatiens of different stars that the separation of light begins te be 
visible as high as 40° or 50° of altitude, 1 was led to believe that 
the dispersive power of the atmosphere must be sufficient, in 
many cases, to produce considerable effect on astronomical ob- 
servations ; and, consequently, to suppose that it would be de- 
sirable to ascertain, if possible, the exact degree of separation 
of the several rays t. 
With this view, therefore, I began a series of observations ; 
the result of w! hich, and the manner of conducting them, I shall 
now take the liberty of laying before the Society. 
* Query. May not this circumstance explain why oaeures though less 
brilliant, bears magnifying better than Jupiter and Venus} 
+ Dr. Herschel, in a note to his paper on Double Stars, published in 
the seventy-fifth volume of the Philesophical ‘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 everlooked in delicate and 
low observations: he gives the measure of two diameters of ¢ Sagittarii, 
which seem to indicate that the refraction of the extreme Tays 1s about 
gy x the mean refraction. I think it due to that great astronomer to 
menuon the circumstance, though it was totally unknown to me till long 
-after I had completed my observations on Mars, 
C3 The 
