8s Dr. Herschel on the Power of 



It is remarkable that, from very different principles, I have 

 formerly determined the length of the visual ray of my 20-feet 

 telescope upon the stars of the milky way, so as to agree nearly 

 with the calculations that have been given.* The extent of 

 what I then figuratively called my sounding line, and what now 

 appears to answer to the power of penetrating into space, was 

 .shewn to be not less than 415, 461, and 497 times the distance 

 of Sirius from the sun. We now have calculated that my tele- 

 scope, in the Newtonian form, at the time when the paper on 

 the Construction of the Heavens was written, possessed a power 

 of penetration, which exceeded that of natural vision 61,18 times; 

 and, as we have also shewn, that stars at 8, 9, or at most 10 

 times the distance of Sirius, must become invisible to the eye, 

 we may safely conclude, that no single star, above 489,551, or 

 at most 61 2 times as far as Sirius, can any longer be seen in 

 this telescope. Now, the greatest length of the former visual 

 ray, 497, agrees nearly with the lowest of these present numbers, 

 489 ; and the higher ones are all in favour of the former com- 

 putation ; for that ray, though taken from what was perhaps 

 not far from its greatest extent, might possibly have reached to 

 some distance beyond the apparent bounds of the milky way : 

 but, if there had been any considerable difference in these deter- 

 minations, we should remember that some of the data by which 

 I have now calculated are only assumed. For instance, if the 

 opening of the iris, when we look at a star of the 7th magnitude, 

 should be only one-tenth of an inch and a half, instead of two, 

 then a, in our formula, will be= 1,5 ; which, when resolved, 



• Phil. Trans. Vol. LXXV. page 247, 248. 



