458 OCULAR SPECTRA. Sect. XL. io. i. 



of the eye, while the fcattered rays from the external blue paper 

 produce a direct fpectrum of that colour on the central part of 

 the eye, inftead of thefe parts of the retina falling reciprocally 

 into their reverfe fpeclra. In exp. d. the colours being the re- 

 verfe of each other, the fcattered rays from the exterior object 

 falling on the central parts of the eye, and there exciting their 

 direct fpeclrum, at the fame time that the retina was excited in- 

 to a reverie fpectrum by the central object, and this direct and 

 reverfe fpectrum being of fimilar colour, the fupevior brilliancy 

 of this fpectrum was produced. In exp. e. the effect of various 

 quantities of ftimulus on the retina, from the different refpec- 

 tive fizes of the internal and external areas, induced a fpectrum 

 of the internal area in the centre of the eye, combined of the 

 reverfe fpectrum of that internal area and the direct one of the 

 external area, in various fhades of colour, from a pale green to 

 a deep blue, with fimilar changes in the fpectrum of the exter- 

 nal area. For the fame reafons, when an internal bright object 

 was fmall, as in exp. f. inftead of the whole of the fpectrum of 

 the external object being reverfe to the colour of the internal 

 object, only a kind of halo, or radiation of colour, fimilar to 

 that of the internal object, was fpread a little way on the exter- 

 nal fpeclrum. For this internal blue area being fo fmall, the 

 fcattered rays from it extended but a little way on the image of 

 the external area of yellow paper, and could therefore produce 

 only a blue halo round the yellow fpectrum in the centre. 



If any one mould fufpect that the fcattered rays from the ex- 

 terior coloured object do not intermix with the rays from the 

 interior coloured object, and thus affect the central part of the 

 eye, let him look through an opaque tube, about two feet in 

 length, and an inch in diameter, at a coloured wall of a room 

 with one eye, and with the other eye naked ; and he will find, 

 that by {hutting out the lateral light, the area of the wall feen 

 through a tube appears as if illuminated by the funfliine, com- 

 pared with the other parts of it; from whence arifes the ad- 

 vantage of looking through a dark tube at diftant paintings. 



Hence we may fafely deduce the following rules to deter- 

 mine before-hand the colours of all fpectra. I. The direct 

 fpectrum without any lateral light is an evanefcent reprefenta- 

 tion of its object in the unfatigued eye. 2. With fome lateral 

 light it becomes of a colour combined of the direct fpectrum 

 of the central object, and of the circumjacent objects, in pro- 

 portion to their refpective quantity and brilliancy. 3. The re- 

 verfe fpectrum without lateral light is a reprefentation in the 

 fatigued eye of the form of its objects, with fuch a colour as 

 would be produced by all the primary colours, except that of the 



object. 



