4-8 Mr. Galloway*s Translation ofDv. Olbers's 



it appeared pale and comparatively dim, it shone forth again 

 with great splendour in 1682 ; and the fainter appear- 

 ances of 1607 and 1759 may be explained by its position in 

 those years in respect of the earth and the sun, without sup- 

 posing it to have suffered any actual diminution. Perhaps 

 we are only ignorant where the comets, in accomplishing their 

 wide revolutions, again recover the splendour they lose when 

 near their perihelia. 



I should mention, however, that Messier, who endeavoured 

 to follow every comet as long as possible, was not able to ob- 

 serve Halley's comet in 1759 after the 4th of June, when its 

 distance from the sun was only about 1*68, and from the earth 

 1'42. It cannot be denied that, according to the commonly 

 received theory, this comet in February and March 1835 will 

 be 4 or 5 times less illuminated, and the intensity of its light 

 about 30 times less than it was on the 4th of June 1759. 

 But at that time the comet set almost in the evening twilight ; 

 at least when the twilight had become sufficiently feeble, it was 

 very near the horizon: and Messier likewise made use of very 

 common telescopes. In March 1835, when the evening twi- 

 light has completely disappeared, the comet will be still high 

 up in the sky, and may be sought for with refracting or re- 

 flecting telescopes of far greater optical power. That Hal- 

 ley's comet, as the experience of 1822 and 1832 has un- 

 doubtedly shown to be the case with Encke's, will be visible 

 at a much greater distance from the sun and the earth before 

 it has passed its perihelion than after, is a position which I will 

 not take upon me to maintain*. 



* This very interesting and remarkable property of Encke*s comet has 



not hitherto, so far as I am aware, been sufficiently attended to. According 



to theory, the intensity of the light of a celestial body not self-luminous, is 



M 

 = fT^-r^» where R and D denote the distances from the sun and the earth, 



and M depends on the magnitude and nature of the individual body. 



When M is constant, the intensity of the light is proportional to C=p^ p.^. 



Pons discovered Encke's comet in 1818 when C = 0*936. Afterwards, when 

 its place had been previously computed by Encke, astronomers were able 

 to find it in 1825 and 1828 when C had a much smaller value. At its dis- 

 covery in 1805, C was = 7'26, and then it was seen with the naked eye, 

 and appeared equal to a star of the 4th magnitude. But after it had passed 

 the perihelion, Rumker, in 1822, lost sight of it when C was = 15'18. On 

 its reappearance in 1832, when the value of C was= 12*12, two observers, 

 Henderson and Mossotti,describeits light as being veryfeeble; andMr.Hen- 

 derson could neither see it with the naked eye nor in the telescope, when 

 C was still := 7*97, and consequently greater than in 1805 when it appeared 

 equal to a star of the 4th magnitude. It would seem that the effect of the 

 sun's rays in the inferior pan of the orbit is to dilute so greatly the light 

 vapours of which this comet seems to be entirely composed, that the ex- 

 terior particles become invisible, and even the parts nearer the centre of 



