498 Penetration into Space by Tele/copes. 



Since, therefore, the quantity of light thrown out by any luminous body Is truly repre- 

 fented by C N, and that an objeft is bright in confequence of light thrown out, we may fay 

 that brightnefs is truly defined by C N. If however, there fliould at any time be occafion 

 for diftinftion, the brightnefs arifing from the great value of C, may be called the intrinfic 

 brightnefs ; and that arifing from the great value of iV, the aggregate brightnefs ; but the 

 abfolute brightnefs, in all cafes, will ftill be defined hy C N. 



Hitherto we have only confidered luminous objects, and their condition with regard to 

 throwing out light. We proceed now to find an expreflion for their appearance at any 

 afligned diflance ; and here it will be proper to leave out of the account, every part oi CN 

 which is not applied for the purpofe of vifion. L reprefencing the whole quantity of light 

 thrown out by CN, we fhail denote that part of it which is ufed in vifion, either by the eye 

 or by the telefcope, /. This will render the conclufions that may be drawn hereafter more 

 unexceptionable ; for, the quantity of light / being fcattered over a fmall fpace in propor- 

 tio'n to L, it may reafonably be looked upon as more uniform in its texture ; and no fcruples 

 about its inequality will take place. The equation of light, in this prefent fenfe, therefore, 

 is CNzzL 



Now, fince we know that the denfity of light decreafes in the ratio of the fquares of the 

 diftances of the luminous objects, the expreflion for its quantity at the diftance of the ob- 



ferver D, will be -^• 



In natural vifion, the quantity / undergoes a confiderable change, by the opening and 

 contradting of the pupil of the eye. If we call the aperture of the iris a, we find that in 

 different perfons it differs confiderably. Its changes are not eafily to be afcertained ; but 

 we fhall not be much out in ftating its variations to be chiefly between i and 2 tenths of an 

 inch. Perhaps this may be fuppofed under-rated ; for the powers of vifion, in a room 

 completely darkened, will exert themfelves in a very extraordinary manner. In fome ex- 

 periments on light, made at Bath, in the year 1780, I have often remarked, that after flay- 

 ing fome time in a room fitted up for thefe experiments, where on entering I could not 

 perceive any one objeft, I was no longer at a lofs, in half an hour's time, to find every 

 thing I wanted. It is however probable that the opening of the iris is not the only caufe of 

 feeing better after remaining long in the dark ; but that the tranquillity of the retina, which 

 is not diflurbed by foreign objedls of vifion, may render it fit to receive impreffions fuch as 

 otherwife would have been too faint to be perceived. This feems to be fupported by telef- 

 copic vifion } for it has often happened to me, in a fine winter's evening, when, at mid- 

 night, and in the abfence of the moon, I have taken fweeps of the heavens, of four, five, or 

 fix hours duration, that the fenfibility of the eye, in confequence of the exclufion of light 

 from furrounding objefts, by means of a black hood which I wear upon thefe occafions, has 

 been very great ; and it is evident, that the opening of the iris would have been of no fer- 

 vicc in thefe cafes, on account of the diameter of the optic pencil, which, in the 20 feet 



telefcope, 



