VOL. LVII.J PHILOSOPHICAL TRANSACTIONS. 439 



of a much longer duration : this sufficiently appears from the well-known expe- 

 riment of a lighted body whirled round in a circle, which needs not make many 

 revolutions in a second, to appear as one continued ring of fire. Hence then it 

 is not improbable that the number of the particles of light, which enter the eye 

 in a second of time, even from Sirius himself, may not exceed 3 or 4 thousand ; 

 and from stars of the 2d magnitude, they may therefore probably not much ex- 

 ceed 100. Now the apparent increase and diminution of the light, which we 

 observe in the twinkling of the stars, seems to be repeated at not very unequal 

 intervals, perhaps about 4 or 5 times in a second : why may we not then suppose 

 that the inequalities, which will naturally arise from the chance of the rays coming 

 sometimes a little denser and sometimes a little rarer, in so small a number of 

 them as must fall upon the eye in the 4th or 5 th part of a second, may be suf- 

 ficient to account for this appearance ? An addition of 2 or 3 particles of light, 

 or perhaps of a single one on 20, especially if there should bean equal deficiency 

 out of the next 20, would, he supposes, be very sensible: this seems at least pro- 

 bable from the very great difference in the appearance of stars, whose light is 

 much less different than people are in general aware of; the light of the middle- 

 most star in the tail of the Great Bear does not, he thinks, exceed the light of the 

 very small star next to it, in a greater proportion than that of about 1 6 or 20 to 

 ] ; and Bouger tells us, in his Traite d'Optique before-mentioned, that he finds 

 a difference in the light of objects of one part in 66 sufficiently distinguishable. 



It will perhaps be objected that the rays coming from Sirius are too numerous 

 to admit of a sufficient inequality, arising from the common effect of chance, 

 so frequently as wouldiae necessary to produce this effect, whatever might hap- 

 j;en in respect to the smaller sLars ; but till wc know what inequality is necessary 

 to produce this effect, we can only guess at it, either one way or the other. 

 There is however another circumstance, that seems to concur in the twinkling of 

 the stars, besides their brightness, and this is a change of colour. Now the red 

 and blue rays being very much fewer, he apprehends, than those of the inter- 

 mediate colours, and therefore much more liable to inequality from the common 

 effect of chance, may help very much to account for this phenomenon, a small 

 excess or defect in either of these making a very sensible difference in the colour. 



It will now naturally be asked, why the frequency of the changes of brightness 

 should not be often much greater, as well as sometimes less, than that above- 

 mentioned, and why the interval of the 4th or 5th, or some such part, should 

 be pitched on rather than the 40th or 50th part of a second, or than a whole 

 second, &c. for, according to the length or shortness of the time assumed, the 

 changes that will naturally occur, from the effect of chance, will be smaller or 

 greater in proportion to each other. The answer to this question will, he thinks, 

 tend to render the above solution more probable, as well as to throw a good deal 



