66 



NA TURE 



[Nov. 22, 1877 



addressed to Dr, Jannetaz, Sdcrdtaire-g^n^ral, rue des 

 Grands Augustins, 7, Paris, France; and all moneys sent 

 to Dr, Bioche, at the same address. 



THE MODERN TELESCOPE 



THE gain to astronomy from the discovery of the 

 telescope has been twofold. We have first, the gain 

 to physical astronomy from the magnification of objects, 

 and secondly, the gain to astronomy of position from the 

 magnification, so to speak, of space, which enables minute 

 portions of it to be most accurately quantified. 



Looking back, nothing is more curious in the history of 

 astronomy than the rooted objection which Hevel and 

 others showed to apply the telescope to the pointers and 

 pinnules of the instruments used in their day ; but doubt- 

 less we must look for the explanation of this not only in 



the accuracy to which observers had attained by the old 

 method, but in the rude nature of the telescope itself in 

 the early times, before the introduction of the micrometer ; 

 the modern accuracy has been arrived at step by step. 



Fig. I. — A portion of the constellation Gemini seen with the naked eye. 



Let us see what the telescope does for us in the 

 domain of that grand physical astronomy which deals 

 with the number and appearances of the various bodies 

 which people space. 



. Fig. 2. — The same region, as seen through a large telescope. 



Let us, to begin with, try to see how the telescope helps 

 us in the matter of observations of the sun. The sun is 

 about 90,000,000 of miles away ; suppose, therefore, 

 by nieans of a telescope reflecting or refracting, whichever 

 we like, we use an eyepiece which will magnify say 900 

 times, we obviously bring the sun within 100,000 miles of 

 us ; that is to say, by means of this telescope, we can 

 observe the sun with the naked eye as if it were within 

 100,000 miles of us. One may say, this is something, but 

 not too much ; it is only about half as far as the moon is 

 from us. But when we recollect the enormous size of the 

 sun, and that if the centre of the sun occupied the centre 

 of our earth the circumference of the sun would extend 

 considerably beyond the orbit of the moon, then one must 

 acknowledge we have done something (to bring the sun 

 within half the distance of the moon. Suppose for looking 

 at the moon we use on a telescope a power of 1,000, that 

 is a power which magnifies 1,000 times, we shall bring the 

 moon within 240 milts of us, and we shall be able to see 



the moon with a telescope of that magnifying power pretty 

 much as if the moon were situated somewhere in 

 Lancashire — Lancaster being about 240 miles from 

 London. 



It might appear at first sight possible in the case of all 

 bodies to magnify the image formed by the object-glass 

 to an unlimited extent by using a sufficiently powerful eye- 

 piece. This, however, is not the case, for as an object is 

 magnified it is spread over a larger portion of the retina 

 than before ; the brightness, therefore, becomes diminished 

 as the area increases, and this takes place at a rate equal 

 to the square of the increase in diameter. If, therefore, 

 we require an object to be largely magnified we must pro- 

 duce an image sufficiently bright to bear such magnifica- 

 tion ; this means that we must use an object-glass or 

 speculum of large diameter. Again, in observing a very 

 faint object, such as a nebula or comet, we cannot, by 

 decreasing the power of the eye-pie :e, increase the bright- 

 ness to an unlimited extent, for as the power decreases, 



