How little effect chromatic aberration plays in the observation of planetary detail may be judged from the fact that all the observers at Flagstaff preferred a neutral-tinted glass to a monochromatic one.
The action of a shade in bringing out detail appears to be somewhat as follows: In viewing a point of light through a telescope of a given aperture, the first minimum of the curve of diffraction, or the middle of the first dark ring, will always be at a given distance from the point of light, but the spurious disk will fade away, out of sight, before it reaches the minimum; and the fainter the point of light, the smaller the spurious disk. As the point of light approaches invisibility in the telescope, the spurious disk approaches zero. Now suppose we consider the light bordering a dark line on Mars as made up of numberless points of light. These points of light are excessively faint, compared with points of light on the sun, or with the light from a star. Their spurious disks are therefore extremely small, so that very little light spills over on to the dark markings; and that is the reason we are able to make out such fine detail on the planet. Now, although small, the spurious disks of these numberless points do diffuse some light on the fine dark markings. By using a shade, we decrease the light from these points, and thus reduce the size of their spurious disks. Therefore less light falls on the dark markings, and the sharpness of their edges is increased.
It is further found at Flagstaff that diaphragming the aperture increases the seeing. Langley, in his article on soaring birds, has shown that there are constant small changes of velocity "within the wind." Now these pulsations must cause waves of rarefaction and condensation, which may be represented as an irregular wave curve, sweeping past the objective. This will cause the planet to swing in the field of the telescope, as the rays are refracted by a layer of denser or rarer air. Now it is evident that the smaller the aperture of the objective, the less variation of the curve will there be in front of the objective at any given instant, or the more homogeneous the air in the path of the rays entering the eye at any given moment. So that, though a smaller objective will not diminish the swinging of the planet in the field, it will diminish the blurring within the planet, and help bring out the detail. Thus, the smaller the objective, the better the seeing, other things being equal. In practise the best results were obtained with a 12-inch diaphragm, as below this the loss of light and of effects due to increasing the size of the spurious disk began to be more powerful factors than the advantages gained from better seeing.
So importantly essential are the shaded glass, and the diminution of the aperture to the study of Martian detail at Flagstaff, that without these aids it would be excessively difficult to make out any of the fine detail on the planet.
It is a mistake to suppose that an observer who has a very keen sight for a small star will necessarily be a good observer of planetary
