he petrous
process.

122 A small opening in the bone at this place is called the _fenestra
rotunda_.

123 Consult some work on physics on the different kinds of lenses and
their uses.

124 With respect to its adjustments the eye does not differ in principle
from various other optical instruments, such as the microscope,
telescope, photographer's camera, etc., which, in their use, form
images of objects. These all require some adjustment of their parts,
called focusing, which adapts them to the distance. The eye's method
of focusing, however, differs from that of most optical instruments,
in that the adjustment is brought about through changes in the
curvature of a lens.

125 The converging power of convex lenses varies as the curvature--the
greater the curvature, the greater the converging power.

126 An oculist is a physician who specializes in diseases of the eye.

127 Some of the more common symptoms of eye strain are nervousness,
headache, insomnia, irritations of the eyelids, sensitiveness to
bright light, and pain in the use of the eyes.

128 Pyle, _Personal Hygiene_.

129 "An infectious disease is one in which disease germs infect (that
is, invade) the body from without. Among the infectious diseases are
some that are quite directly and quickly conveyed from person to
person and to these the term contagious is applied. Formerly a sharp
line was drawn between infection and contagion, but to-day it is
recognized that no such line exists."--HOUGH AND SEDGWICK, _The
Elements of Hygiene and Sanitation._

130 The arctic explorer, Nansen, states that during all the time that
his party was exposed to the low temperature of the arctic region,
no one was attacked by a cold, but on returning to a warmer climate
they were subject to colds as usual. The difference he attributes to
the absence of germs in the severe arctic climate. There seems to be
no doubt but that most of our common colds are due to attacks of
germs.

131 An interesting biological fact is that the female _Anopheles_, and
not the male, sucks the blood of animals and is the cause of the
spreading of malaria.

132 The habit of spitting upon the floors of public buildings and street
cars, and also upon sidewalks, is now recognized as a most dangerous
prac