together were oppofite to each other, as yellow and blue, red and green, &c. according to the table of reflections and tranfmiffions of light in Sir Ifaac Newton's Optics, B. II. Fig. 3. the fpectra of thofe colours were of all others the moft brilliant, and beft defined; because they were combined of the reverfe fpectrum of one colour, and of the direct fpectrum of the other. Hence, in books printed with fmall types, or in the minute graduation of thermometers, or of clock-faces, which are to be seen at a distance, 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 used 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 spectrum of the letter is the fame colour with the ground on which they are feen, the unfteadiness 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, whose spectra are thus reciprocally fimilar to each colour, is owing to this greater ease that the eye experiences in beholding them distinctly; and it is probable, in the organ of hearing, a fimilar circumftance 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 sensations 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 easy to magnify it to tenfold its fize: for if, when the fpectrum is formed, you still keep your eye fixed on the filk area, and remove it a few inches further from you, a green circle is feen round the red filk; for the angle now fubtended by the filk is lefs than it was when the fpectrum was formed, but that of the spectrum continues the fame, and our imagination places them at the fame diftance. Thus when you view a spectrum on a sheet of white paper, if you approach the paper to the eye, you may diminifh it to a point; and if the paper is made to recede from the eye, the spectrum will appear magnified in proportion to the distance. I was furprised, and agreeably amused, with the following experiment. 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 fitting with my back to the fun, fixed my eyes for a minute exaly on the centre of the letter N in the middle of the word; after Vol. I. Mм m clofing clofing my eyes, and fhading them fomewhat with my hand, the word was diftinctly feen in the fpectrum in yellow letters. on a blue field; and then, on opening my eyes on a yellowish wall at twenty feet distance, the magnified name of BANKS appeared written on the wall in golden characters. 6. Conclufion. Ir was obferved by the learned M. Sauvages (Nofol. Method. Cl. VIII. Ord. 1.) that the pulfations of the optic artery might be perceived by looking attentively on a white wall well illuminated. A kind of net-work, darker than the other parts of the wall, appears and vanishes alternately with every pulfation. This change of the colour of the wall he well afcribes to the compreffion of the retina by the diastole of the artery. The various colours produced in the eye by the preffure of the finger, or by a ftroke on it, as mentioned by Sir Ifaac Newton, feem likewife to originate from the unequal preffure on various parts of the retina. Now as Sir Ifaac Newton has fhewn, that all the different colours are reflected or transmitted by the lamina of foap bubbles, or of air, according to their different thickness or thinness, is it not probable, that the effect of the activity of the retina may be to alter its thicknefs or thinnefs, fo as better to adapt it to reflect or tranfmit the colours which ftimulate it into action? May not mufcular fibres exift in the retina for this purpofe, which may be lefs minute than the locomotive muscles of microfcopic animals? May not thefe mufcular actions of the retina conftitute the fenfation of light and colours; and the voluntary repetitions of them, when the object is withdrawn, conftitute our memory of them? And lastly, may not the laws of the fenfations of light, here investigated, be applicable to all our other fenfes, and much contribute to elucidate many phenomena of animal bodies both in their healthy and difeafed ftate; and thus render this investigation well worthy the attention of the phyfician, the metaphyfician, and the natural philosopher? November 1, 1785. Dum, Liber! aftra petis volitans trepidantibus alis, Me quoque, me currente rota revolubilis ætas END OF THE FIRST PART. |