May 31, 1873.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
949 
THE COLOUR OP CANTHARIDES. 
BY HENRY POCKLINGTON. 
The colours and colouring matter of the thorax, 
head, and other portions of cantharides that bear the 
metallic green lustre so characteristic of these insects, 
agree so closely in origin and kind with those that 
formed the subject of my last paper* that it will be 
convenient to discuss the colouring matters without 
reference to any particular portion of the insect. As 
with the wing cases the colours of the thorax, etc., are 
in large part due to interference and in part to fluor¬ 
escence. The persistent green colour referred to hi my 
previous paper has been the subject of the later inves¬ 
tigation of which I now propose to give the results. 
Many readers of my former article have expressed 
their regret that it did not include any account of the 
mode of working followed in these inquiries, and have 
also intimated a desire that a more graphic method of 
charting the spectra should be followed in the paper, 
that non-spectroscopists may be able to follow the 
description better ; I will endeavour as briefly as 
possible to meet their wishes in both respects. 
As stated in my former paper, the instruments of 
which I make use, are a Sorby Browning micro¬ 
spectroscope, a small direct vision spectroscope 
adapted, for use with the microscope, and capable 
of showing two spectra at once, and side by side to 
permit comparison; and a stand-spectroscope of 
moderate dispersion, giving a much longer spectrum 
than the micro-spectroscope, and working, on this 
account partly, rather better when bands occur in 
the blue or extreme green. A very simple and 
inexpensive arrangement will enable any of my 
readers to see absorption bands, and thus enable 
such of them as are not acquainted with the subject 
to understand what sort of things they are. Having 
closed the shutters of a room looking south, make a 
narrow slit in the shutter and place before it a small 
prism (the triangular pendant of a chandelier will do 
if nothing better be at hand, or failing this, three 
pieces of glass may be fastened into a cork so as to 
form a triangular bottle, and this, filled with water, 
or carbon bisulphide, will make a very good make¬ 
shift prism) with its thin edge at right angles to the 
length of the slit, and pointing downwards. A more 
or less pure spectrum will be projected on the opposite 
wall, or may be allowed to fall upon a white paper 
screen. If a bottle filled with dilute solution of the 
permanganate of potash be placed outside the slit so 
that the sunlight passes through the bottle, then 
through the slit and prism, the spectrum will be seen 
to be no longer “continuous,” that is, the various 
u prismatic ” colours will be no longer uniformly 
shaded the one into the other from red to violet, but 
there will be seen four (or five if the arrangements 
be very good) black bands crossing the spectrum, and 
the violet will be entirely missing. These are absorp¬ 
tion bands, the spectrum being “ discontinuous,” with 
general absorption of the violet. These bands are, of 
course, due to the absence of certain colours that are 
absorbed by the solution in the bottle, and the rays 
of light that our senses translate into the colours 
being missing when the sunlight (or lamp-light) falls 
upon the prism, the prism simply disperses those that 
are present into their proper places in the spectrum, 
and leaves the other places blank, and the absence of 
light meaning darkness (that is, blackness) we have 
* Pharmaceutical Journal, 3rd Ser., Yol. III. p. 681. 
Third Series. No. 153. 
a spectrum of such colours as the incident light was 
composed of, crossed by black band spaces due to the 
absence of such colours as the incident light had been 
deprived of. That they are bands is due to the fact 
that the prism paints an infinite number of images of 
the slit, each image being of one colour, and when all 
colours are present, so exactly side by side that the 
most cunning eye cannot discern the piecing, any 
absent colours being represented by black portraits 
of the slit, their number side by side determining 
the width of the absorption band. 
_ The production of the spectrum is due to the 
different amount of refraction that each constituent 
colour of white light suffers in passing through the 
prism, and this is connected with the wave-length of 
the rays in question, the short waves being most 
retarded in passing through the prism, and therefore 
the most refracted, and the long or red waves (those 
that vibrate least rapidly) the least retarded ; thus 
each wave of the infinite number that composes the 
spectrum is differently affected by the prism. Out 
of this law arises the wonderful modern method of 
scientific inquiry known as spectrum analysis. 
The subject is one much too large for discussion here, 
and has only been introduced to this extent to clear 
the atmosphere of a misapprehension induced by an 
error in Mr. Stoddart’s interesting paper on “ Spectral 
Analysis adapted to Pharmacy,” in this Journal for 
September, 1869, in which Mr. Stoddart, by a remark¬ 
able slip of the pen, refers the prismatic dispersion to 
this cause, that, “ the red rays of the spectrum vibrate 
so weakly that they can only penetrate the thin edge 
of the prism. Those of greater intensity are capable 
of penetrating the thicker portions of the glass, and 
are thereby refracted at a greater angle.” * 
To meet the other desideratum I have charted the 
spectrum of the cantharides previously described in 
addition to the spectra of the new ones. 
The green portions of the insects were carefully 
pulverized and macerated in cold ether, alcohol, and 
bisulphide of carbon for forty-eight hours. The solu¬ 
tion was poured off and examined in a narrow test- 
tube placed before the slit of the instrument. In the 
other field was placed the interference spectrum, 
crossed by its twelve black bands, and the position 
of the absorption bands read off against these. If it 
be desired to chart them, a piece of paper may be 
ruled with faint pencil lines to correspond with the 
bands of the interference spectrum, and the absorp¬ 
tion bands easily and exactly drawn to correspond 
with them. This mode of charting is exceedingly 
easy and permits of the greatest accuracy. If thought 
desirable, the position of Fraunhofer’s lines may be 
compared with the interference bands and drawn on 
the chart to facilitate comparison by those who have 
not the Sorby plate. Before beginning work, and at 
frequent intervals, it is desirable to calibrate the in¬ 
strument by comparing the sodium yellow bright line 
spectrum (there is only one bright “ D ” line in these 
small spectra) with the interference spectrum, to 
make sure that the two spectra lie truly side by side, 
otherwise the spectra will be inaccurately charted. 
The spectrum given by the simple solution of the 
colouring matter having been carefully charted, the 
* A moment’s reflection, of course, leads one to see the 
fallacy of this. The sun shines as white light through the 
thickest plate-glas3 window equally with the thinnest of 
crown. Those who desire to pursue the subject may 
refer to G-anot’s ‘Physics’ or Deschanel’s ‘Natural Philo¬ 
sophy.’ 
