May 13, 1898.] 



SCIENCE. 



655 



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 be- 

 lieves to be the only obstacle (though for- 

 tunately it is to be overcome) which can 

 interfere with his studies of Martian archi- 

 tecture. 



So far we have considered only what 

 great telescopes cannot accomplish, 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 rea- 

 son 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 re- 

 search, and render possible many investi- 

 gations 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 Tele- 

 scopes Pay ?' which was evidently not writ- 

 ten by one of those to whom reference has 

 just been made, but by one of another class, 

 whose known acquaintance with astronom- 

 ical work would tend to give his opinion 

 considerable weight with many intelligent 

 readers. In discussing the subject it was 

 seriously asked whether the great invest- 

 ments of money which had been made in 

 the giant instruments of the latter half of 

 the nineteenth century had been attended 

 by commensurate advances in astronomical 

 knowledge. The question is certainly one 

 that deserves serious consideration, for it 

 would surely be poor policy to erect great 

 telescopes if they are no better than smaller 

 and much cheaper ones. It is desirable, 

 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 instruments in use 

 are refractors, it will be seen that our prob- 

 lem is, for the most part, a comparison of 

 the properties of a large achromatic lens 

 with those of a small one. To render the 

 discussion more definite let us compare a 

 40-inch leas of 62 feet focus with a 10-inch 

 lens of 15-?j- feet focus. The large lens, 

 then, has a diameter four times that of the 

 small one, which means that its area is six- 

 teen times as great. It will thus receive 

 upon its surface from a given star sixteen 

 times as much light, and all of this will be 

 concentrated in the point-like image of the 

 star, except that portion which is lost in 

 transmission through the lens. On account 

 of its greater thickness, the large lens trans- 

 mits only about 65 per cent, of the visual 

 rays that fall on it, while the small lens 

 transmits about 77 per cent. But after al- 

 lowance has been made for the loss due to 

 both absorption and reflection it is found 

 that the image of a given star produced by 

 the large telescope will be nearly fourteen 

 times as bright as that given by the small 

 one. In this instance all of the light is 

 concentrated in a point, but in the case of 

 a planet or other extended object, on ac- 

 count of the fact that the focal length of 

 the telescope increases as its aperture in- 

 creases, the brightness of the image is no 

 greater with the large glass than with the 

 small one. The image is, however, four 

 times as large, and this has a most im- 

 portant bearing upon certain classes of 

 observations, particularly in photographic 

 and spectroscopic work. 



There remains still another peculiarity of 

 the large lens as distinguished from the 

 small one. On account of the nature of 

 light, the power that a lens possesses of 

 separating two luminous points which are 

 so close together as to be seen as a single 

 object by the unaided eye depends directly 

 upon its aperture. Thus, if we consider a 



