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THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[March 1, 1873. 
Snell connected the angle of incidence with the angle 
of refraction, by proving that the sine of the one 
divided by the sine of the other is absolutely constant 
for the same medium, whatever the obliquity of the 
incident ray may be. The lines answering to these 
“sines” will be pointed out in the lecture. The con¬ 
stant quotient here referred to is called the index of re¬ 
fraction. The discovery is one of the corner-stones of 
optical science. 
Quickly following Snell’s discovery, is the application 
of it by Descartes to the explanation of the rainbow. 
The bow is seen when the back is turned toward the 
sun. Draw a straight line through the spectator’s eye 
and the sun, the bow is always seen at the same angu¬ 
lar distance from this line. This was the great diffi¬ 
culty. Why should the bow be always, and at all parts, 
41 degrees distant from this line ? Taking a pen and 
calculating the track of every ray through a rain-drop, 
Descartes found that at one particular angle the rays 
emerged from the drop almost parallel to each other; 
being thus enabled to preserve their intensity through 
long atmospheric distance; at all other angles the rays 
quitted the drop divergent, and through this divergence 
became practically lost to the eye. The particular angle 
here referred to was the foregoing angle of 41 degrees, 
which observation had proved to be invariably that of 
the rainbow. 
But in the rainbow a new phenomenon was introduced 
—the phenomenon of colour. And here we arrive at 
one of those points in the history of science when men’s 
thoughts and labours so intermingle that it is difficult 
to assign to each worker his precise meed of honour. 
Descartes was at the threshold of the discovery of the 
composition of solar light. But he failed to attain per¬ 
fect clearness, and it is certain that he did not enunciate 
the true law. This was reserved for Newton, who went 
to work in this way. 
Through the closed window-shutter of a room he 
pierced an orifice, and allowed a thin sunbeam to pass 
through it. The beam stamped a round image of the 
sun on the opposite white wall of the room. In the path 
of this beam Newton placed a prism, expecting to see 
the beam reflected, but also expecting to see the image 
of the sun after refraction, round. To his astonishment 
it was drawn out to an image whose length was five 
times its breadth; and this image was divided into 
bands of different colours. Newton saw immediately 
that this image was due to the fact that some consti¬ 
tuents of the solar light were more deflected by the 
prism than others, and he concluded, therefore, that 
white solar light was a mixture of lights of different 
colours and of different degrees of refrangibility. 
The elongated image here referred to is called the 
spectrum. Newton divided the spectrum into seven 
parts, red, orange, yellow, green, blue, indigo, violet; 
which are commonly called the seven primary or pris¬ 
matic colours. 
This was the first analysis of solar light by Newton ; 
but the scientific mind is fond of verification, and never 
neglects it where it is possible. It is this stern con¬ 
scientiousness on the part of those who pursue it that 
gives adamantine strength to science, and renders all 
assaults on it unavailing. Newton completed his proof 
by synthesis. For instance, he refracted the colours 
back, reblended them, and thus reproduced the white 
light out of which they came. 
In the lecture, Newton’s experiment in Newton’s own 
form is made; it is afterwards made with instruments 
which yield larger and richer effects than those seen by 
Newton. The synthesis of white light is effected in 
three different ways. Firstly, the colours of the spec¬ 
trum are squeezed together and blended by a cylindrical 
lens, white light being the result of their mixture ; se¬ 
condly, an image of the carbon points, whence the light 
issues, is built up from the colours of the spectrum; 
thirdly, in virtue of the persistance of luminous impres¬ 
sions upon the retina, the prismatic colours may be 
mixed together in the eye itself, the impression of white¬ 
ness being the result. The drawing out of the white 
light into a spectrum is called dispersion. And here 
historic completeness renders necessary a brief reference 
to an error of Newton’s. He supposed that refraction 
and dispersion went hand in hand, and that if you 
abolished the one you at the same time abolished the 
other. He maintained this opinion to the end of his 
life, and thus retarded the progress of discovery. Dol- 
land, however, proved that by combining two different 
kinds of glass the colours could be extinguished, still 
leaving a residue of refraction, and he employed this re- 
siclue in the construction of achromatic lenses—lenses 
which yield no colour—which Newton thought an im¬ 
possibility. This point is illustrated in the lecture by 
combining a prism of water and one of glass; the colour 
is destroyed but not the refraction. 
The refraction and dispersion of bisulphide of carbon, 
are compared with those of water, in order to explain 
why the first mentioned liquid is used when our object 
is to obtain spectra of great extent and richness of colour. 
[The spectra exhibited in this part of the lecture, and 
from then till its close, were of great beauty. The one 
upon which the final experiments -were made, by which 
the actual character of colour was explained, must have 
been at least two and a half feet wide, and its length, 
when dispersed by two prisms, covered the whole screen.] 
Having unravelled the interwoven constituents of 
white light, w r e have next to inquire what part the con¬ 
stitution so revealed enables this agent to play in na¬ 
ture F To it we owe all the phenomena of colour ; and 
yet not to it alone, for there must be a certain relation¬ 
ship between the ultimate particles of natural bodies and 
light to enable them to extract from it the luxuries of 
colour. But the function of natural bodies i3 here selec¬ 
tive, not creative. There is no colour generated by any* 
natural body in any kind of form. Natural bodies have 
showered upon them, in the white light of the sun, the 
sum total of all possible colours, and their action is li¬ 
mited to the sifting and appropriating from the total the 
colours which really belong to them, and rejecting those 
which do not. It will fix this subject in your minds if 
I say that it is the portion of light which they reject, 
and not that which belongs to them, that gives bodies 
their colours. 
Let us begin our experimental inquiries here by asking- 
what is the meaning of blackness ? Pass a black ribbon 
in succession through the colours of the spectrum; it 
quenches all. This is the meaning of blackness—it is 
the result of the absorption of all the constituents of 
solar light. Pass a red ribbon through the spectrum. 
In the red light the ribbon is a vivid red. Why ? Be¬ 
cause the light that enters the ribbon is not quenched or 
absorbed, but sent back to the eye. Place the same rib¬ 
bon in the green or blue of the spectrum; it is black as 
jet. It absorbs the green and blue light, and leaves the 
space on which they fall a space of intense darkness.. 
Place a green ribbon in the green of the spectrum. It 
shines vividly with its proper colour; transfer it to the 
red, it is black as jet. Here it absorbs all the light that 
falls upon it, and offers mere darkness to the eye. When 
white light is employed, the red sifts it by quenching 
the green, and the green sifts it by quenching the red, 
both exhibiting the residual colour. Thus the process 
through which natural bodies acquire their colours is a 
negative one. These colours are caused by subtraction, 
not by addition. The action of various liquids and 
solids upon the spectrum is also illustrated; some cutting 
off the one end, others cutting off the other end, and 
some selecting for absorption the middle of the spectrum. 
These experiments prepare us for the consideration of a 
point regarding which error has found currency for ages. 
You will find it stated in books that blue and yellow 
lights mixed together produce green. They do not. 
Blue and yellow are complementary colours and produce 
