SECT. XL. io. OCULAR SPECTRA. 637 



5. Variation offpeflra in refpeft to dijlinftnefs andjlze ; 

 with a new way of magnifying objefts. 



i. It was before obferved, that when the two co- 

 lours viewed together were oppofite to each other, 

 as ydlow and blue, red and green, &c. according 

 to the table of reflections and tranfmiflions of light 

 in Sir Ifaac Newton's Optics, B. II. Fig. 3. the fpec- 

 tra of thofe colours were of all others the moft bril- 

 liant, and beft defined ; becaufe they were combined 

 of the reverfe fpectrum of one colour, and of the 

 direct fpectrum of the other. Hence, in books print- 

 ed with fmall types, or in the minute gradation of 

 thermometers, or of clock-faces, which are to be 

 feen at a diftance, if the letters or figures are colour-, 

 ed 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 fpec- 

 trum of which is fitnilar 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 pro- 

 duce 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 reciprocally fimilar to 

 each colour, is owing to this greater eafe that the eye 

 experiences in beholding them diftin&ly ; and it is 

 probable, in the organ of hearing, a fimilar circum- 

 ftance may conftitute 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 



