Scct. XL. 10. 5. OCULAR SPECTRA, • '5 



together were oppofite to each other, as yellow and blue, red 

 and green, &c. according to the table of reflections and tranfmif- 

 fions of light in Sir Ifaac Newton's Optics, B. II. Fig. 3. the 

 fpectra of thofe colours were of all others the mod brilliant, and 

 bed defined ; becaufe they were combined of the reverfe fpec- 

 trum of one colour, and of the direct fpectrum of the other. 

 Hence, in books printed with fmall types, or in the minute grad- 

 uation of thermometers, or of clock-faces, which are to be {cai. 

 at a diftance^ if the letters or figures are coloured with orange, 

 and the ground with indigo ; or the letters with red, and the 

 ground with green ; or any other lucid colour is ufed for the 

 letters, the fpectrum of which is fimilar to the colour of the 

 ground *, fuch letters will be feen much more diftinctly, and 

 with lefs confufion, than in black or white : for as the fpectrum of 

 the letter is the fame colour with the ground on which they are 

 feen, the unfteadinefs of the eye in long attending to them will 

 not produce coloured lines by the edges of the letters, which is 

 the principal caufe of their confufion. The beauty of colours 

 lying in vicinity to each other, whofe fpectra are thus recipro- 

 cally fimilar to each colour, is owing to this greater eaie that 

 the eye experiences in beholding them diftinctly ; and it is prob- 

 able, in the organ of hearing, a fimilar circumflance may confti- 

 tute the pleafure of melody. Sir Ifaac Newton obferves, that 

 gold and indigo were agreeable when viewed together ; and 

 thinks there may be fome analogy between the fenfations of light 

 and found. . (Optics, Qu. 14.) 



In viewing the fpectra of bright objects, as of an area of red 

 filk of half an inch diameter on white paper, it is eafy to mag- 

 nify it to tenfold Its fize : for if, when the fpectrum is formed, 

 you (till keep your eye fixed on the lilk area, and remove it a 

 few inches further from you, a green circle is feen round the 

 red filk ; for the ansde now fubtended bv the filk is lefs than 

 it was when the fpectrum was formed, but that of the fpectrum 

 continues the fame, and our imagination places them at the 

 fame diftance. Thus when you view a fpectrum on a (licet of 

 diite paper, if you approach the paper to the eye, you may di- 

 minifii it to a point ; and if the paper is made to recede from 

 the eye, the fpectrum will appear magnified in proportion to the 

 diftance. 



I was furprifed, and agreeably amufed, with the following ex- 

 periment.- I covered a paper about four inches fquare with 

 yellow, and with a pen filled with a blue colour wrote upon the 

 middle of it the word BANKS in capitals, as in fig. 5, and fit- 

 ting with my back to the fun, fixed my eyes for a minute cxa 

 on the centre of the letter N in the middle of. the word ; after 



Vol. I. M M m cloOi 



