THE PSYCHOLOGY OF TELESCOPIC VISION 



By W. ALFRED PARR. 



Little Buttercup, of "H.M.S. 'Pinafore' " fame, 

 gave vent to a profound piece of general philosophy 

 when she assured Captain Corcoran, of that vessel, 

 that " Things are seldom what they seem." Now, 

 although this is a fundamental verity familiar enough 

 to us all, we are occasionally reminded of the 

 peculiar virtue underlying this Gil- 

 bertian dictum in quite an unexpected 

 manner when looking through a tele- 

 scope. If we regard a far-off land- 

 scape through a powerful telescope, 

 fitted with an erecting day eyepiece, 

 we soon become aware of the fact 

 that any distant cottages or houses 

 which our landscape may contain 

 appear to us strangely out of drawing. 

 More especially is this the case with 

 such houses as happen to have their 

 longest sides parallel to our line of 

 sight, so that we look along them, as 

 it were, instead of squarely at them. 

 We shall find, on examining these 

 closely, that, instead of presenting 

 the ordinary aspect we are accustomed 

 to associate with the rules of fore- 

 shortening, they boldly bid defiance 

 to all our ideas of perspective and 

 stand out at us in a manner which is 

 truly astonishing. And, if we recall 

 the statement we so often hear, that 

 the function of a telescope is to make 

 a far-off object appear as if it were 

 only at a short distance from us, our 

 astonishment will be the greater, since 

 no houses within our previous 

 experience wear such a curiously 

 distorted guise as these. 



It was claimed by the pro- 

 moters of the giant telescope, 

 which was intended to form the clou of the 

 Paris Exhibition in 1900, that the moon was to 

 be brought by this instrument literally within our 

 grasp ! — if we could reach a metre — and popular 

 accounts of the great Yerkes telescope affirmed that 

 it would make our satellite appear just as it would 

 be seen with the naked eye if it were suspended but 

 sixty miles over our heads. Such irresponsible 

 statements as these are not only in the highest 

 degree misleading in themselves, as I ventured to 

 point out some years ago in a letter to the British 

 Astronomical Association,* but, even supposing the 

 exaggerated claims, just quoted, as to the " approxi- 

 mating " power of a great telescope to be possible of 



realization, it can still be shown that the telescopic 

 image of an object is vastly different from the actual 

 aspect that object would present if viewed by the 

 naked eye at the same apparent distance. This 

 somewhat ignored fact is a distinct and important 

 feature of telescopic vision ; for it has an interesting 

 bearing on our interpretation of tele- 

 scopic pictures in general, be they 

 astronomical or terrestrial. 



Now, our surest way of keeping 

 this peculiar action of the telescope in 

 mind is to remember that, far from 

 even apparently bringing distant 

 objects nearer, as the popular claim 

 has it, the telescope in reality merely 

 enlarges the naked-eye view. It 

 brings to our notice objects which, by 

 reason of their great distance from us, 

 would otherwise remain beyond the 

 limits of critical vision, and thus 

 enables us to see them under conditions 

 different from those under which our 

 experience is usually gained. In other 

 words, the telescopic image and the 

 naked-eye image are essentially copies 

 of one another on varying scales, and 

 it is only in our different interpretation 

 of these scales that the peculiarity 

 of telescopic vision resides. Distant 

 objects are habitually seen under a 

 small visual angle, and they conse- 

 quently fail to excite our critical 

 notice in the same degree as do 

 near objects which are seen under a 

 larger visual angle. But when 

 the telescope comes to our aid 

 and enlarges our naked-eye view 

 for us, we are apt to fall into 

 error, and, if we do not in reality 

 consider ourselves suddenly confronted with an 

 actually nearer object, we, at any rate, regard that 

 object with all the inferences which the enlarged 

 image conjures up in our minds. We mentally 

 endow the magnified retinal image with all the 

 attributes which an object of its apparent size would 

 possess, and fancy our larger picture also nearer, in 

 conformity with our past association of largeness and 

 nearness. We must remember, however, that in 

 looking through a telescope we become aware of 

 conditions which are foreign to our usual experience, 

 and of which, but for the instrument's power, we 

 should have remained in ignorance. 



Thus, to consider for a moment the simple 



See Journal Brit. Astro. Assoc, Vol. IX, p. 271. 



