104 
much as two or three hundred kilometers’— 
how, we can not say, but probably in no 
manner different from that in which lesser 
disturbances are propagated. And this is 
typical of a fact the teacher should not 
overlook. Scientific progress is the discov- 
ery of more truth, rather than the contra- 
diction of laws already clearly established. 
No one who has had the pleasure of see- 
ing some of the beautiful slides, such as 
those from photographs by Professor Foley, 
showing sound waves in all stages of de- 
velopment—birth, growth, reflection, re- 
fraction—ecan question their interest, or 
their instructiveness. And yet, what are 
the emotions that stir one as he reads of 
the visible sound waves described by sev- 
eral observers after moments of extra vio- 
lent cannonading along the battle line.® 
Distinct bands were actually seen moving 
across the clouds with the known velocity 
of sound, or again, equally distinct against 
a clear sky. Such points of interest should 
eseape no teacher of physics, for nothing 
can be more legitimate than enlisting the 
pupil’s interest with illustrations of this 
kind. 
Other problems in other fields are fully 
as numerous and as fascinating as those we 
have noted, but we will have to content 
ourselves with mere reference to some of 
them. What of the application of optical 
principles? At home we have protective 
lighting® based: upon an ever-improving ap- 
preciation of the correct principles of il- 
luminating engineering. Then again, 
American manufacturers have struggled 
heroically and with incomplete success to 
produce good optical glass. The student 
wants to know the reason for this endeavor 
7 Science Abstracts A, No. 657, 1917. 
Abstracts, A, No. 658, 1917. 
8 L’Astronomie, July, 1917. Scientific American, 
November 10, 1917, p. 343. 
8 Electrical World, May 18, 1918, p. 1049. 
Science 
SCIENCE 
[N. S. Vou. XLVIII. No. 1231 
—the reason for the failure—the necessity 
for such glass anyway. And who is to en- 
lighten him if not the physics teacher. 
When he seeks to learn something about 
the submarine camera, he comes upon the 
stabilizing gyroscope.’° He also finds that 
the search lamp is an optical instrument of 
high design, and that the manufacture and 
operation of the portable gasoline-motor- 
electric-generator search-lamp outfits em- 
braces a wide field of activity and many 
physical principles.1+ 
: Even the teacher must be alert to keep 
up with only such phases of physical de- 
velopment due to the war as are published. 
There are new methods for testing mirrors — 
for signalling purposes and special appa- 
ratus for measuring magnifying powers 
and the angle between the axes of binocu- 
lars. ‘The Michelson interferometer finds 
use in finishing prisms, lenses and combina- 
tions. There are weathering tests for glass, 
new tests for parallax in the telescopic 
sights for rifles, new methods for determin- 
ing the illumination and field of view of 
field glasses, special parallelism tests, mi- 
crometers for measuring prism angles be- 
fore polishing, and new surface testing 
methods. The refractometry and identifi- 
cation of glass requires special equipment 
and training.4? Range finders have multi- 
plied, even the prismatic binocular having 
joined the list by the simple addition of a 
calibrated disk by means of which the dis- 
tance to an object of known dimensions can 
be determined.‘ Unfortunately all of these 
10 Scientific American, May 19, 1917, p. 483. 
Trans. Illum. Eng. Soc., XII., 8, November 20, 
1917, p. 396. 
11 Trans. Illum. Eng. Soc., XII., 8, November 20, 
1917, p. 357. Elect. World, December 9, 1916, p. 
1169. 
12 Science Abstracts, A, 379, 1917. 
13 Scientific American Supp., No. 2178, Septem- 
ber 29, 1917, p. 198. 
14 Scientific American, January 12, 1918, p. 60. 
