33^ -D r . Darwin 4 * Experiments on the 
being reverfe to the colour of the internal objedt, only a kind 
of halo, or radiation of colour, fimilar to that of the internal 
objedt, was fpread a little way on the external fpedtrum. For 
this internal blue area being fo fmall, the fcatterea 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 fpedtrum in the center. 
If any one fhould fufpedt that the fcattered rays from the 
exterior coloured object do not intermix with the rays from the 
interior coloured objedt, and thus affedt the central part of the 
eye, let him look through an opake 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 funfhine, com- 
pared with the other parts of it ; from whence arifes the advan- 
tage of looking through a dark tube at diftant paintings. 
Hence we may fafely deduce the following rules to determine 
before-hand the colours of all fpedtra. i. The diredt fpedtrum 
without any lateral light is an evanefcent reprefentation of its 
objedt in the unfatigued eye. 2. With fome lateral light it 
becomes of a colour combined of the diredt fpedtrum of the 
central objedt, and of the circumjacent objedts, in proportion 
to their refpedtive quantity and brilliancy. 3. The reverfe 
fpedtrum without lateral light is a reprefentation in the fatigued 
eye of the form of its objedts, with fuch a colour as would 
be produced by all the primary colours, except that of the ob- 
jedt. 4. With lateral light the colour is compounded of the 
reverfe fpedtrum of the central objedt, and the diredt fpedtrum 
of the circumjacent objedts, in proportion to their refpedtive 
quantity and brilliancy, 
7 
II. 
