20G Dr Pearson, On the use of large telescopes [March 20, 



ness than one, and yet they are visible to a greater distance be- 

 cause they excite more attention. In like manner a distant ship, 

 hardly visible on the horizon at night, is easily discerned when 

 magnified five or six times by the sailor's night glass:" and speak- 

 ing of the Galilean telescope or opera-glass : " The apparent bright- 

 ness of an object seen through the instrument is nearly the same 

 as that which it presents to the naked eye, except that more or 

 less of the light is reflected or dispersed at each surface of the 

 glasses through which it has to pass." Speaking of the Astro- 

 nomical telescope, he says that " The quantity of light condensed 

 into each point of the image, and consequently the apparent 

 brightness of the field, depends, cceteris paribus, on the aperture 

 of the object-glass, and the transparency of the material of which 

 it is formed," and again, p. 29, " In order to derive full benefit 

 from the use of a telescope with any given eye-piece, the emergent 

 pencil should just fill the pupil of the eye, in which case the appa- 

 rent brightness of an object seen through the telescope would be 

 nearly equal to that with the naked eye, as mentioned in the case 

 of the opera-glass. The diameter of the object-glass should there- 

 fore be equal to that of the pupil of the observer's eye multiplied 

 by the magnifying power." The same principle is laid down in 

 Schmidt's Optik, S. 528 where, speaking of a refracting telescope 

 at Dorpat with an object glass of 4-| inches, it is calculated that, 

 assuming the semi-diameter of the pupil of the eye to be yVth 

 of an inch, the clearness of the object is undiminished up to a 

 magnifying power of 54 : but that after that it diminishes rapidly, 

 being with a magnifying power of 480 only ^th part of what it 

 was: and as a rule of geometrical optics, I find no exception 

 taken to the principle itself, the younger Herschel (Encycl. Metrop. 

 vol. iv. p. 396, art. Light) meeting the case of stars of the 

 smallest magnitude, in the following way by saying that " While 

 generally the apparent intrinsic brightness of the image must 

 be less than that of the object, this supposes the object to have 

 a sensible magnitude : but when both the object and the image 

 are physical points, the eye judges only of absolute light, and 

 the light of the image is therefore proportional to the apparent 

 magnitude of the lens as seen from the object. In the case of a 

 star for instance, the absolute light of the image is simply as the 

 square of the aperture, and this is the reason why stars can be seen 

 in large telescopes which are too small to be seen in small ones." 

 I have reproduced these extracts, not so much to remove any 

 doubts as to the law itself, as to show how clear and defined a law 

 we have to face when we read of the phenomenon viewed at Wash- 

 ington, entirely confirmed as it is by what is mentioned above of 

 "ships seen through a glass at night," and by Herschel's account, 

 published only in 1800, of what he had noticed first at Bath in 



