12S THE FUNCTION OF LARGE TELESCOPES. 



brain, and still remain unknown to the telescope to which they were 

 ascribed. An influential newspaper selected these latter observations 

 as the text of an editorial setting forth the marvelous benefits the 

 Yerkes telescope is destined to confer upon mankind. 



It may be added that the great telescope of the "moon hoax "is 

 hardly more extravagant in conception than certain schemes which have 

 been proposed in all seriousness within the past year. One of these 

 inventors, whose familiarity with the difficulties of telescopic observa- 

 tion is certainly surpassed by his optimism, remarks : " I think the limit 

 (of magnification) will be due to the shaking of the instrument caused 

 by the trembling of the earth and of the clockwork mechanism which 

 moves the telescope. Under these high magnifications extremely mi- 

 nute vibrations are so much magnified that a small object like that of a 

 house upon the surface of Mars would dart in and out of the field of 

 vision so as to prevent its being photographed." And this he believes 

 to be the only obstacle (though fortunately it is to be overcome) which 

 can interfere with his studies of Martian architecture. 



So far we have considered only what great telescopes can not accom- 

 plish, and were I not to pass rapidly on to some positive statements of 

 another character I might be supposed to believe that they have no 

 reason for existence, or at best are no better than small ones. But I 

 shall endeavor to show that exactly the contrary is true; that while 

 large telescopes do not possess the extraordinary powers conferred upon 

 them by fertile imaginations, they nevertheless play a most important 

 part in scientific research, and render possible many investigations 

 which are altogether beyond the reach of smaller instruments. It seems 

 the more necessary to dwell upon this point, for only a few years ago 

 there appeared in print an article entitled "Do large telescopes pay?" 

 which was evidently not written by one of those to whom reference has 

 just been made, but by one of another class, whose known acquaintance 

 with astronomical work would tend to give his opinion considerable 

 weight with many intelligeut readers. In discussing the subject it was 

 seriously asked whether the great investments of money which had been 

 made in the giant instruments of the latter half of the nineteenth cen- 

 tury had been attended by commensurate advances in astronomical 

 knowledge. The question is certainly one that deserves serious con- 

 sideration, for it would surely be poor policy to erect great telescopes 

 if they are no better than smaller and much cheaper ones. It is desir- 

 able, therefore, to point out, if I can, some of the elements of superiority 

 of large instruments which seem to me to make them worth all that 

 they cost and more. 



Leaving aside reflecting telescopes, as most of the very costly instru- 

 ments in use are refractors, it will be seen that our problem is, for the 

 most part, a comparison of the proprieties of a large achromatic lens 

 with those of a small one. To render the discussion more definite let 

 us compare a 40-inch lens of 62 feet focus with a 10-inch lens of 15£ feet 



