98 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[July 30, 1870. 
.glass into a rarer medium, air, it is again bent, 
but this time away from the dotted perpendicular, 
and therefore takes the course R. By means, there¬ 
fore, of such a piece of glass, which is called a prism, 
the light is brought round a corner. 
10. If a beam of ordinary light refracted by a 
prism, be received on a sheet of paper or other sur¬ 
face, it presents the appearance of a stripe of colours. 
In the last diagram it will be seen that the direction 
of the refracted ray I—R is always towards the base 
or broad side BC of the prism. When the stripe of 
•colours, or ‘ spectrum’ of ordinary light is received 
on the screen, it will be noticed that the order of the 
coloured bands is— 
Red-Orange-Yellow-Green-Blue-Indigc-Violet 
and that the violet end is that which is nearest to 
the base of the prism, the red nearest to the opposite 
angle A. The violet is said therefore to be the most 
refrangible (i. e. capable of being most bent by re¬ 
fraction), the red, the least refrangible, and the other 
colours intermediate in the order given. 
11. It was by an experiment of this kind, and by 
reasoning upon it, that Newton was led to the ex¬ 
planation of the constitution of ordinary sunlight. 
White light is made up of a number of coloured (and 
also some invisible) rays, each possessing a different 
•degree of refrangibility. When these pass through 
a prism, they are bent to the extent peculiar to each 
respectively, and so undergo dispersion, and become 
perceptible as separate and distinct components; when 
they are received simultaneously in the eye, without 
previously submitting to this separation, their com¬ 
bined effect is ‘ whiteness.’ That white light is thus 
constituted is shown by the experiment already de¬ 
scribed, and also by the circumstance that if the band 
of separated coloured rays, produced by sending a 
beam of white light through a prism, be made to pass 
through another prism formed of the same material 
and in a position the reverse of the first, white light 
will be re-generated. To effect this properly, the 
two prisms must have an angle A (see diagram) of 
the same magnitude in both, so that their refracting 
surfaces may be parallel, when the base of the one 
is applied to the opposite angle of the other. Under 
these circumstances the rays which are separated 
by the first prism are reunited by passing through 
the second. 
12. We are now in a position to understand the 
production of colour in natural objects. A surface is, 
for example, red, when it decomposes the light which 
falls upon it, reflecting only the constituent which 
gives the impression we call redness, and absorbing 
all the rest. For a similar reason, on looking through 
a piece of glass, it appears to be of the colour of the 
rays which it transmits; the remaining constituents 
may be reflected or absorbed. Knowledge of these 
facts helps also to the understanding of the changes 
of hue which coloured objects exhibit when illumi¬ 
nated by light from different sources. When, for 
example, a piece of silk appears mauve by daylight, 
it gives the impression which a mixture in due pro¬ 
portions of red and blue gives. By gaslight the same 
usually appears red or reddish, from the fact that 
there is hi gaslight so much less of the blue consti¬ 
tuent than in sunlight, that when illuminated by the 
former, the proportion of blue reflected by the silk 
must be much less, and the red effect predominates. 
In a photograpliic chamber illuminated by nearly 
pure yellow light, all but yellow and white objects 
appear black; that is, there is no light which they 
are capable of reflecting. 
13. It has already been mentioned that the con¬ 
stituents of sunlight include certain rays which are 
invisible. They do not produce the impression of 
light, but the one set of them is capable of heating; 
the other of effecting chemical decomposition and 
combination. 
If a beam of sunlight is passed through a prism, 
and the resulting spectrum received upon a screen, 
it may be found, by the aid of delicate thermometers, 
or a thermo-electric pile, that the position of greatest 
heating power is in the dark beyond the extreme 
point where the red is visible. The position of 
greatest luminosity is in the yellow portion of the 
band of light of which the visible spectrum consists; 
whilst the power of bringing about chemical changes 
is confined to the other end, the maximum point be¬ 
ing found between the indigo and violet. It will, of 
course, be understood that not only do the luminous 
parts of the spectrum merge into one another by im¬ 
perceptible gradations, but that the three forms of 
action, resulting in heat, light, and chemical acti¬ 
vity, are not confined to one narrow portion, but 
each extends over a considerable proportion of the 
total length of the spectrum. However, the heating 
rays are no longer found on reaching the extreme 
violet; and, in like manner, the rays possessing 
photographic powers do not extend into the yellow 
and red. 
In a rough sort of way, the subjoined diagram will 
indicate the relative positions of the constituents of 
sunlight after they have been separated by the prism. 
Dark—R—0—Y—G—B—I—V—Dark 
\ __ / 
Luminous. 
x -v.-" 
Heating. _ 
Chemical. 
Ox Diet and Regimen in Sickness and Health. By 
Horace Dobell, M.D. Fourth Edition. Rewritten 
and much Enlarged. London: H. K. Lewis. 1870. 
We have perused, with considerable pleasure, this 
small volume in its new form. It contains a number of 
tables of analysis of various articles of food, dietary scales, 
and alcohol percentages, also numerous recipes; direc¬ 
tions for ventilating, heating, disinfecting, and cleansing; 
together with many practical suggestions for the preser¬ 
vation of health, the prevention of infection and disease, 
and the management and comfort of a patient and of the 
sick-chamber. 
Although much of the matter is old, many of the sug¬ 
gestions familiar, and most of the rules laid down those 
of common and every-day practice, yet the collection of 
these in a form easy of reference, is of some value, and 
the book cannot fail to be of interest to the general 
reader. 
A new chapter, consisting of a letter and a leading 
article from the ‘ Times,’ on “ London Noise and London 
Sleep,” is worthy of a passing notice, and the author de¬ 
serves some credit for having brought the subject pro¬ 
minently before the public. 
There are few, whether residents in or visitors to Lon¬ 
don, whose indignation has not been roused simultaneously 
with their attention, by two “cabbies careering down op¬ 
posite gutters and holding a conversation across the road 
