to Heal and lilumlnait Oojc6ti. ^ij 



■ix-iH feiifibly affeft the focal length of objetft-glaflla ; but in 

 compound vlfion, fuch as in a niicrofcope, where a vcryfmall 

 lens is made to caft a lengthened fecondary focus, this dif- 

 ference becomes ftill more confidcrable. 



By an attentive and repeated infpedibn, I found that my 

 object was very well feen in red, better in orarfge, and dill 

 better in yelloVv; full as well in gretnj but to lofs advantage 

 in blue ; indifferently well in indigo, and with more imper- 

 leftion in violet. 



The trial was made upon one of the niicrofcopic objeiils 

 which are generally prepared for tranfparent vifion; but as £ 

 iifed it in the opaque wjly, I thought that others might be 

 chofen which would anfwer the purpofe better; andj in order 

 to give fonie variety to my experiments, and to fee the etfecit 

 which differently coloured fubltances might have on the ravs 

 of light, [ provided the following materials to be viewed: — red 

 paper, green paper, a piece of brafs, a nail, a guinea, black 

 paper. Having alfo found that a higher power might be 

 uledj with fufficient convenience for the ravs of light to come 

 from the piifm to the objeft, I made the microfcope magnify 

 42 times. 



The appearance of the nail in the microfcope is fo beau- 

 tiful, that it deferves to be noticed; and the more fo, as it i» 

 accompanied with circumftances that are very tavourable for 

 an invedigation, fuch as that which is under our prefent con- 

 lideration. I had chofen it on account of its folidity and 

 blacknefs, as being molt likely to give an impartial refult of 

 the modifications arifing from an illumination by difiTcrenily 

 coloured rays; but, on viewing it, I was ftruck with the fight 

 of a bright conftellation of thoufands of luminous points fcat- 

 tered over its whole extent, as far as the field of the micro- 

 fcope could take it in. Their light was that of the illumi- 

 nating colour, but differed confuierably in brightnefs; fome 

 of the points being dim and faint, while others were lumi- 

 nous and brilliant. The brightefl of them alfo admitted of 

 a. little variation in their colour, or rather in the intcnfity of 

 the fame colour; for in thecentre of ibme of the nioli. brilliant 

 of thcfe lucid appearances, their light had more \lvacity, and 

 fteuifd to deviate from the illuminating tint to-Aaidi whitc- 



liefi| 



