FEBRUARY 8, 1901. ] 
mated pictures may be mounted, a great 
variety of interesting work may be per- 
formed. Professor Merritt* and I have 
shown ina recent paper that very beautiful 
photographs of the manometric flame are 
obtainable with the aid of such an instru- 
ment, and Professor Hallock has made use 
of asimilar method in an extended analysis 
of the human voice. Any one who has the 
patience to systematically study and inter- 
pret records of this sort, might add greatly 
to our knowledge of the physics of articu- 
late speech. With the same instrument 
the motions of vibrating strings and rods 
and the decadence of overtones with the 
time may be studied. The instrument is, 
indeed, applicable to a very great variety 
of short-time phenomena, such, for example 
as the duration of exposure obtained by 
various flash powders, a subject of which at 
present our knowledge is very incomplete. 
Still another interesting line of work 
within the reach of physics teachers in our 
secondary schools consists of the photog- 
raphy of the infra-red spectrum. Bec- 
querel showed many years ago that the flu- 
orescence of calcium sulphide and other 
substances could be checked and almost 
annihilated by the long waves of the spec- 
trum, and Fomm, a student of von Lommel, 
in 1890 succeeded in photographing the 
infra-red spectrum of the sun by placing a 
fluorescent screen in the spectral image 
and subsequently making a contact print 
by laying the sereen face to face with an 
ordinary sensitive dry plate. In this way 
he was able to identify and determine the 
wave lengths of numerous dark lines in the 
spectrum. The absorption spectra of chlo- 
rophyll, of water, of the salts of didymium, 
samarium, erbium and of other substances 
which possess well-marked bands in the 
visible spectrum, have been mapped in this 
way by Becquerel, but his work should be 
* Nichols and Merritt, Physical Review, Vol. VII., 
p. 93. 
SCLENCE. 
207 
repeated since his estimation of wave- 
lengths is known to be entirely at fault. 
The number of substances as yet untested 
is very great and the accurate investigation 
of them might lead to results of high im- 
portance. The method of direct photog- 
raphy by means of plates sensitized for 
the infra-red is probably to be preferred for 
such work, to the troublesome use of the 
fluorescent screen. 
Finally, not to extend this list of feasible 
experiments further, permit me to remind 
my fellow teachers of physics that we have 
in the spectro-photometer, an instrument 
by means of which one may investigate 
visible radiation, reflecting power and the 
transmission of light, by all sorts of sub- 
stances. It is a great convenience in 
such work to have at hand the very per- 
fect modern instruments designed by Lum- 
mer and Brodhun or by Brace. I am 
aware that none of our school laboratories 
are likely to contain such apparatus; but 
it is only necessary to purchase a Vierordt 
slit and to adapt the same to any good or- 
dinary spectroscope in order to be in posi- 
tion to do good spectro-photometric work. 
I have attempted in this paper to mention 
only a few of the numerous topics of re- 
search available for physics teachers. It 
is one of the characteristics of our science, 
that every contribution to our knowledge 
brings with it a group of further problems 
to be solved. One can not read intelligently 
any memoir describing experimental work 
without perceiving the possibility of ex- 
tending the investigation further. The one 
essential requisite to the carrying out of 
the suggestions thus received is that burn- 
ing desire to try things for one’s self which 
characterizes the investigator. Such a de- 
sire is the fruit of that habit of experimen- 
tation to which I have already referred, a 
habit which I deem it the first duty of every 
one, who has the ambition to lay claim to 
the title of man.of science, to foster and 
