TRANSACTIONS OF SECTION A. 94] 
Although Newton admitted light into his darkened room in this way for certain 
experiments, yet he was perfectly aware that the coloured image which is formed 
when such a beam is passed through a prism consists of innumerable coloured 
circles (‘Opticks,’ Prop. iv., Prob. 1, 1), which overlap, and that the mixture 
of the heterogeneous rays can be diminished by making the circles of smaller 
diameter. The first method which he proposed for doing this was to have the 
hole at a great distance from the prism, so that rays coming from the centre only 
of the sun would be used. 
Then follows this passage: ‘But that those circles may answer more distinctly 
to that hole, a lens is to be placed by the prism to cast the image of that hole 
upon the paper. If this be done it will not be necessary to place that hole very 
far off; no, not beyond the window.’ 
He then describes fully how this is to be carried out practically. 
This passage is followed by another, which, strange to say, has escaped the 
notice of Brewster and many others, ‘ Opticks,’ p. 59, 2nded. ‘ Yet instead of the cir- 
cular hole (F), ’tis better to substitute an oblong hole shaped like a long parallelo-. 
gram, with its length parallel to the prism (A, B, C). For if this hole be an inch 
or two long, but a tenth or twentieth part of an inch broad, or narrower, the light 
of the image “?,e. the spectrum ” (pt) will be as simple as before or simpler, and 
the image will become much broader and, therefore, more fit to have experiments. 
tried in its light than before.’ 
He also says that a triangular aperture may be used, the base of the triangle 
being about the tenth of an inch, its height an inch or more. In the spectrum 
formed by light passing through such an aperture, he says that the bases of the 
triangular images overlap a little, but their vertices do not. 
In all these experiments the prisms were placed at the angle of minimum 
deviation. The lens was generally placed at a distance from the aperture equal 
to double its focal length, and the screen at the same distance from the lens. 
The prisms are carefully described; one is stated to have ‘had some veins 
running along witbin the glass from one end to the other, which scattered some 
of the sun’s light irregularly.’ Others are described as being ‘free from veins.’ 
He also used prisms filled with rain-water. In one of his letters to Oldenburg, 
Horsley’s ‘ Newton,’ vol. iv. p. 343, he refers to a crystal prism. This must have 
been rock crystal. 
Very ordinary prisms will show the Fraunhofer lines. Prof. Rood using the 
prisms of an ordinary candelabrum found no prism among twelve which did not 
show several lines. It seems now impossible to account with certainty for their 
not being seen by Newton; it certainly was not for the reason given by Brewster. 
Nor probably was it due to the inferior quality of the glass or workmanship of 
the prisms. Newton used an assistant for observing the spectra in certain experi- 
ments. ‘Opticks,’ p. 110 (2nd ed.) ‘ An assistant, whose eyes for distinguishing 
colours were more critical than mine, did by right lines,’ . . . ‘drawn across the 
spectrum, note the confines of the colours.’ It is possible that this assistant may 
haye seen the lines, but Newton’s attention was not called to them, and their 
existence was not recorded by him. 
For full details of many of Newton’s experiments it is necessary to refer to the: 
‘Lectiones Optic,’ which were not published until after Newton's death. They 
were delivered in the years 1669-71, at Cambridge, where the original manuscript 
was deposited. 
In one of the early lectures he describes his observation of the spectrum of 
the planet Venus. ‘The object glass of a seven-foot telescope, its aperture being 
two inches and more broad, to transmit a sufficient quantity of rays,’ received the 
light of the planet, and formed upon a paper at the distance of seven feet an 
image ‘like a lucid point.’ A prism being then interposed, the spectrum formed is. 
described as ‘a very fine line, though not very bright, however very easily to be 
discerned.’ He then remarks: ‘And I believe the same thing might be observed 
of stars of the first magnitude, as of Sirius, especially if a lens be used four or six. 
inches broad, that it may transmit many rays.’ In a subsequent page it is stated. 
that this experiment had been successfully tried, 
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