1 b"t) An Essay on a Properly in Light 



both these colours will be distinctly marked upon the retina, be- 

 cause the direct rays from each are strong enough to drown the 

 secondary rays from the other ; and even if the ground was black, 

 which reflects no light, instead of blue, these secondary rays from 

 the red colour will not be perceived, because they are not suffi- 

 ciently strong to overpower the other light which strikes upon the 

 same parts of tiic retina. It may be necessary here to observe, 

 that what we call darkness and black, is not in general the total 

 absence of light, but the contrast between those bodies that re- 

 flect a great deal, and those that reflect only a small quantity of 

 it. A man, on going out of day-light into a dungeon, fancies 

 the darkness is absolute, u'hilc the persons residing there are 

 capable of distinguishing one another very easily: and again, a 

 man, on coniingout of a dark dungeon, finds the glare of day- light; 

 as painful to his eves as the direct light of the sun is to any one 

 else. 1 mention this, because, otherwise it might appear to the 

 reader a contradiction to suppose that a black spot could be re- 

 presented on the retina, every part of which is covered with a 

 coat of light. 



3. Philosophers have taken a great deal of pains to find out 

 what the tail of a comet is, and how it is produced ; but not one 

 of them, that I have ever heard of, has thought it necessary to 

 explain in what manner this light is brought down to the eye. 

 \Vhether a comet's tail be produced by electricity, or any other 

 cause (unless it be ignited gas, like the flame of a candle, and 

 then it would put on a very different form), it is evidently light 

 passing across the eye, and not towards it, and can only he visi- 

 ble by sending down its secondary rays according to the principle 

 I have supposed ; for, otherwise, if it was refracted by any sub- 

 stance, either surrounding tlie comet or in our atmosphere, the 

 tail to be visible must be pointed towards us; and if it was re- 

 flected, it would be seen in confu'iion ; for it is impossible that 

 particles of light reflected at random by particles of any fluid, 

 could put on so regular and constant a form as the comet's tail 

 always assumes. 



In order to show why the comet's tail is always opposite the 

 sun, it is necessary to show what it is ; and to do this I must clear 

 the way by showing that the opinions generally entertained con- 

 cerning a comet's tail are erroneous. 



Sir Isaac Newton supposed that the tail was somehow or other 

 produced by the heat of the sun ; that the reason that it was al- 

 ways turned from the sun was, because the comet passed through 

 the sun's atmosphere; and that this ignited vapour, or whatever 

 it is, was lifted up by the sun's atmosphere in the same maimer 

 as smoke, vapour and other light substances are by our atmo- 

 sphere. 



