206 
made in his theory of elasticity holds true. 
Measurements, however, showed widely 
varying values for Poisson’s ratio and led to 
important modifications of the theory. All: 
this may be thought to belong to the ancient 
history of the science, but to-day, after 
nearly half a century, exact data are known 
for only a few substances. I do not men- 
tion this as one of the determinations es- 
pecially adapted to the secondary school 
laboratory. The quantity to be observed 
is, of course, exceedingly small. Still the 
method by which I am accustomed to illus- 
trate the phenomenon to my classes; that 
of stretching a glass tube filled with mercury 
and noting the fall of the liquid in the very 
fine capillary neck gives a good result with 
glass, and would probably be adaptable 
without serious difficulties to such other 
materials as can be obtained in the form 
of tubes. With the interferometer, direct 
measurements of the change of diameter 
ought to be readily made but this instru- 
ment is at present not a part of our school 
equipments. 
Another field of work which is easily 
opened to physics teachers in our secondary 
schools is that of the study of flame temper- 
atures. ‘Lhe temperatures of the Bunsen 
burner, the ordinary luminous gas flame, 
the candle flame and the acetylene flame 
are already pretty well established, although 
many important details which are capable 
of being worked out by a patient observer 
are still lacking. When it comes to the 
question of other flames than these, we 
have only the wildest estimates based upon 
measurements made by methods, the in- 
adequacy of which has been abundantly 
demonstrated. The flames of alcohol, of 
ether and of carbon disulphide burning in 
air would afford subjects for an interesting 
and profitable study during one’s leisure 
hours. The apparatus needed for such a 
research consists of a fairly sensitive galva- 
nometer, a resistance box and a standard 
SCIENCE. 
(N.S. Vou. XIII. No. 319. 
cell, together with about one meter each of 
platinum and platinum-rhodium wire. The 
obstacles to even approximately accurate 
high temperature work have until recently 
been almost insuperable on account of the 
difficulty of calibrating the thermo-element 
used; but I have shown in a recent paper * 
how the very elaborate and laborious meth- 
ods of calibration hitherto employed by 
those engaged in such work may be avoided 
without loss of accuracy and how by the 
ingenious method first employed by Wag- 
gener } in the study of the Bunsen burner, 
and subsequently by myself for the measure- 
ment of the acetylene flame, the heat losses 
in the thermo-junction which had vitiated 
the results of earlier observers may be 
eliminated. 
We physics teachers have amused our- 
selves at one time or another, like many 
other people, with photography, and a few 
of us, doubtless, deserve to be classed as 
experts in the fascinating art. Herein lies 
a double opportunity for research; in the 
further development of the science which 
underlies photographic processes and in the 
application of the photographie method to 
the numerous problems in physics to which 
it is especially adapted. Consider, for ex- 
ample, in illustration of the former, the 
fruitful field of inquiry suggested by Pro- 
fessor Nipher’s{ recent memoir upon the 
action of light and of the X-rays on pre- 
viously exposed plates, and of the latter the 
countless investigations of recent years in 
which the photographic plate has been 
utilized for recording and studying the phe- 
nomena of our science. 
By means of a camera containing a re- 
volving drum, upon which a piece of the 
flexible film used in the making of ani- 
* Nichols, Physical Review, Vol. X., p. 324. 
t+ Waggener, Wiedemann’s Annalen, Vol. LYIII., p. 
579. 
{ Nipher, Transactions of the St. Louis Academy of 
Sciences (1900). 
