"You may have on the front of your coat a small stain, or grease-spot, in a position where you could plainly see it, yet might wear the coat for days or even weeks in complete unconsciousness of the existence of the stain until some one pointed it out to you. After that you cannot look at the coat without seeing the stain, and it becomes so persistently obtrusive that you are compelled to have it removed. There is, however, no imagination about your seeing the mark."
John here said to me: "Professor, I noticed you said that many who do not believe in the actuality of the lines and markings on Mars frequently refer to the fact that, while they are stated to be seen through small telescopes, they are quite invisible through a very large instrument, and they regard this as proving that the lines or markings do not exist. Is there not something in this argument?"
"Well, John," I replied, "the argument sounds not only plausible, but reasonable, and inexperienced persons might use the argument, believing it to be a sound and good one. I must, however, confess that I have been surprised to see this argument used by persons who must surely know that there is no weight in it at all.
"It is well known to all practical observers, and indeed to all who have studied optical matters, that, for several reasons, very large telescopes are quite unsuited for the observation of fine planetary detail.
"The real advantage of these enormous instruments lies in their great 'light-grasp,' which enables observers to see very faint points of light, such as small satellites of planets, faint stars, double stars, distant comets, or nebulæ, which could not be seen with a smaller instrument necessarily having less 'light grasp.' Yet this very excess of light, which is the great advantage of a large instrument, is one of the things that spoils the definition of faint planetary details; it drowns them all out, or 'breaks them up.'
"Again, these large instruments are much more liable than smaller ones to what is termed 'chromatic' and 'spherical' aberration; and this also is detrimental to definition. No very large refractor is entirely free from these defects.
"Another objection is that, in using such large and long-focussed instruments, a much higher power must necessarily be employed than in the case of smaller instruments. This high power magnifies all the little movements and disturbances in our atmosphere to exactly the same extent as it magnifies the object looked at, with the result that these disturbances blur out all fine detail. The canal lines on Mars could never be seen in such circumstances. If the object were looked at through a smaller instrument, with lower power, it might be fairly well seen, for the atmospheric disturbances would not be magnified to such an extent as to spoil definition.
"There are very few nights in the year when these immense instruments can be used to advantage on the planets, whilst a smaller instrument might define well three or four nights out of every six. It is on record that the user of Lord Rosse's great reflector stated that there were only about three nights in the year when its best definition could be obtained; and its use has produced very meagre results, compared with what had been anticipated.
"It is also almost universally recognised that in using these great instruments, whether for photography or for the visual observation of fine detail, it is absolutely necessary to stop down the aperture to a very large extent, by reducing it to about 12 inches in diameter or even less. The big telescope is thus really converted into a small one of long focus.
"There is, in addition, the acknowledged fact that nearly every discovery of new detail on planets has been made with a comparatively small telescope, although the same objects may have been under constant observation for years with big telescopes. The new detail was never noticed until after it had been seen with a smaller instrument, and perhaps only then when atmospheric conditions were unusually good.