43 
hope antiquaries will not forget to inquire into the subject, should 
it ever come into their hands. 
According to Dr. Young, " Dr. Wollaston has determined the di- 
vision of the coloured image or spectrum in a more accurate manner 
than had been done before: by looking through a prism at a narrow 
line of light, he produces a more effectual separation of the colours 
than can be obtained by the common method of throwing the sun's 
image on a wall. The spectrum formed in this manner consists of 
four colours only, viz. red, green, blue, and violet." 
" The colours differ scarcely in their quality within their respec- 
tive limits, but they differ in brightness; the greatest intensity of 
light being in that part of the green that is nearest the red ; a narrow- 
line of yellow is generally visible at the limits of the red and green, 
but its breadth scarcely exceeds that of the aperture by which the 
light is admitted; and Dr. Wollaston attributes it to the mixture of 
the red with the green light, which is found to be the same, what- 
ever refracting powers may have been used for its formation." 
" If the breadth of the aperture viewed through a prism is in- 
creased, the space occupied by each variety of light in the spectrum 
IS augmented in the same proportion, and each portion encroaches 
on the neighbouring colours and is mixed with them ; so that the 
red is succeeded by orange, yellow, yellowish green, and on the other 
side by violet, and it is in this state the prismatic spectrum is com- 
monly exhibited." Thus there are five colours to this specimen of 
Dr. Young's common prismatic spectrum, four positive ones to 
Dr. Wollastou's, and five doubtful ; but seven to Sir Isaac Newton's. 
The doctor proceeds to observe, that " when a beam of light is so 
much enlarged as to exceed the angular magnitude of the spectrum, 
it retains its whiteness in the centre, and is terminated by two difl^er- 
ent series of different colours at the different ends. These series are 
still divided by well marked lines ; on the one hand, the red remains 
unmixed; the space belonging to the green and blue becomes a 
greenish yellow nearly uniform throughoiit, and here the appearance 
of the colour ends, the place of the violet being scarcely distinguish- 
G 2 
