THE PLANET *MARS IN FEBRUARY 1869. 
47 
disc, beyond a doubt. He used darkening glasses perfectly gra- 
duated from end to end, and by this means was enabled to obtain 
the most accurate estimate of the relative brilliancy of various 
parts of the disc. 
But the greater darkness of Jupiter’s disc near the limb is 
what was theoretically to have been expected. An opaque 
globular body directly illuminated by a distant luminous orb 
should appear brightest in the centre of its disc ; because the real 
illumination diminishes as the angle at which the light rays 
meet the surface diminishes, and the apparent brilliancy at any 
point of an object is always equal to the real illumination at the 
point. 
In the case of Mars then, the apparent illumination of dif- 
ferent parts of the disc, varies in a manner which is directly the 
reverse of what was theoretically to be expected. Therefore, it 
behoves us to determine with so much the greater accuracy 
whether the eye may not be deceived in this as in the former 
case. I believe the experiment applied by Mr. Browning to 
Jupiter’s disc, has never been applied to that of Mars. But, 
fortunately, a series of photometrical experiments by Dr. Zollner, 
although not directed to the question we are considering, but to 
the determination of the total amount of light received from 
Mars at different epochs, yet affords a satisfactory reply to our 
doubts. For it will be easily understood that when a globe is 
not illuminated strictly according to the usual law — but, from 
some reason unknown, presents an anomalous variation of bril- 
liancy — the total amount of light received from it at different 
times will not correspond with the estimate deduced according 
to the usual law. For example: the moon’s light at full does not 
bear to the moon’s light at the quarter the proportion which 
would exist if the moon were a perfectly smooth globe, and 
therefore illuminated strictly according to the law mentioned 
above (in dealing with Jupiter), and by assuming — what is 
practically the case — that the illumination of the hemisphere of 
Mars turned towards the sun varies according to some law 
depending merely on the distance from the central point of that 
hemisphere, it follows that, by noting the amount of light 
received from Mars at different times — and especially by com- 
paring the amount received from him in quadrature, with that 
received when he is in opposition — it becomes possible to deduce 
the law according to which different parts of his disc are illumi- 
nated. For although when Mars is in quadrature his gibbosity 
is not very remarkable (see figs. 1 and 3), yet the true centre of 
the illuminated hemisphere is removed a considerable distance 
from the centre of the disc,* and the total illumination is there- 
fore affected in a remarkable manner by the planet’s gibbosity. 
* Its place is marked by a small cross ia figs. 1 and 3. 
