6a8 OCULAR SPECTRA. SECT. -XL. jo, 



with the other parts of it ; from whence arifes the 

 advantage 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 fpeclra. 

 j. The direct fpedtnim w'thout any lateral light is 

 an evanefcent repre fen ration of its ohjett in the un- 

 fatigued eye. 2. With fome lateral light it becomes 

 of a colour combined of the dire6t fpetfrnm of the 

 central objeft, and of the circumjacent objects, in 

 proportion to their refpe&ive quantity and brillian- 

 cy. 3. The reverfe fpedrum without lateral light is 

 a reprefentaticn in the fatigued eye of the form of 

 its objects, with fuch a colour as would be pro- 

 duced by all the primary colours, except that of 

 the object. 4. With lateral light the colour is 

 compounded 'of the reverfe fpectrurh of the central 

 object, and the idireft fpe&rurn of the circumjacent 

 objects, in propoition to their refpective quantity 

 and brilliancy. 



2. Variation and vivacity of the jpe&ra occafioned by 

 extraneous light. 



The reverfe fpectrum, as has been before explain- 

 ed, is fimilar to a colour, formed by a combina- 

 tion of all the primary colours, except that with 

 which the e\e lus been fatigued in making the ex- 

 periment : fo the reverfe fpectrum of red is fuch a 

 green as would be produced by a combination of all 

 fhe other prifmatic colours Now it muft be ob- 

 ferved, that this reverfe fpe-flrum of red ia there- 

 fore the direft fpeclrum of a combination of all the 

 other prifmatic colours, except the red; whence, on 

 removing the* eye from a piece of red (ilk to a fheet 

 of white paper, the green fpectrum, which is per- 

 ceived, may either be called the reverfe fpectrum of 



the 



