SCIENTIFIC IMPORTANCE OF X-RAYS—GARLAND 179 
Unfortunately, if you look at them sufficiently long, you will lose 
your eyelashes, and probably your eyebrows, if not most of your hair. 
This is because of the destructive effect of large amounts of X-rays 
on the hair follicles and other living tissues. However, it is safe to 
gaze at the rays for a few minutes, using a small beam (perhaps 1- 
inch diameter). One must sit in total darknes for some 30 minutes 
before he can appreciate the faint fluorescence produced by the rays 
on the human retina; one is then able to see small lead objects placed 
between the X-ray tube and the human eye, and, if one has an opaque 
lens (cataract) or cornea, he can ascertain whether or not the optic 
nerve is intact. The method is used occasionally by radiologists, at 
the request of eye specialists, to assure that the patient has an intact 
retina and nerve before surgical procedures on the cornea or lens are 
performed. It is to be noted that objects will seem upside down, 
since the focusing power of the human lens will be ineffective with 
X-rays. 
The action of X-rays on photographic film is similar to that of light 
rays. Ordinary X-ray films are coated on both sides with a silver 
halide emulsion, especially sensitive to the violet range of the spectrum, 
because they are used mostly with double intensifying screens which 
glow with that color. These screens consist of thin pieces of card- 
board coated with calcium tungstate and a protective transparent 
film. They are placed in a special bakelite or aluminum-fronted 
frame, and for satisfactory results must make perfect contact with 
the film. Special nonscreen films may be used for improved detail, 
but are from 5 to 10 times slower than screen films. The modern film 
has a cellulose-acetate base (with no greater fire hazard than paper). 
The finished roentgenogram is a negative image and is studied in a 
flashed opal glass illuminator. 
The fluoroscopic screen usually is made with zinc sulfide because 
it must carry an image to be viewed directly and this chemical fluo- 
resces in the color range to which the eye is quite sensitive (yellow 
green). Roentgen’s original screen was made of barium platino- 
cyanide, which was much less efficient than the present type. All 
fluoroscopic screens must be covered with lead glass to protect the 
operator from exposure to X-rays. The operator must “adapt” his 
eyes prior to examination by remaining in a darkened room for several 
minutes or by wearing special goggles before beginning the work. 
A beam of X-rays consists of electromagnetic radiations of various 
wave lengths. If of very short wave length they are of high penetrat- 
ing power; if of very long wave length they may penetrate only a few 
millimeters of tissue. The quality of a beam of rays may be measured 
by various methods, the most convenient one in ordinary practice being 
the determination of their absorption by some material such as copper 
