SEPTEMBER 29, 1899. ] 
If, while a person is sittting or standing, 
the head be much inclined, the deviations of 
the settings of the cross hair become much 
greater than when the head is erect. The 
apparently horizontal and the apparently 
vertical lines seem always to be turned out 
of their true position in clockwise direction 
when the head is inclined to the right, and 
in counter-clockwise direction when the 
head is turned to the left. 
At the suggestion of Professor C. E. St. 
- John, some settings were made while the 
observer lay on a horizontal raised board 
with head supported so that the line join- 
ing the eyes was vertical. In this case the 
deviations were enormous, as may be seen 
in the results given in Table III. It will 
be observed that the apparently horizontal 
and vertical lines were in all cases turned 
from their true positions, in clockwise di- 
rection if the observer was lying on his 
right side, and in counter-clockwise direc- 
tion if he was lying on his left side. If, 
after a setting had been made, the observer 
looked away from the instrument and at 
the walls of the room for a little while and 
then back again into the telescope, the 
deviation seemed enormous at first, but 
it decreased rapidly and continuously, and 
at the end of, perhaps, ten seconds the 
setting seemed again good. While making 
these observations the eyes were carefully 
SCIENCE. 
427 
sented, though without its screen, in the 
accompanying figure. The observer, looking 
through a horizontal tube about 35 centi- 
meters in diameter and 2 meters long, 
which shuts out of sight extraneous objects 
and is blackened inside and furnished with 
several diaphragms to prevent reflection 
from the inner surface, sees a white circu- 
lar field 35 centimeters in diameter, divided 
into halves by a fine, straight, black ink 
line. This field is the central portion of a 
large piece of smooth cardboard, mounted 
ona wooden disc, 75 centimeters in diame- 
ter. The disc is in a vertical plane perpen- 
dicular to the line of sight, and can be 
turned about a horizontal axis in the 
geometrical axis (produced) of the observ- 
ing tube. By the use of simple mechanism 
the observer may rotate the disc until the 
line on the cardboard seems to him hori- 
zontal or vertical. Its deviation from true 
horizontality or verticality can then be read 
off, to a hundredth of a degree if such accu- 
racy is ever desirable, by an assistant with 
the help of a microscope and scale on the 
back of the disc. 
While using this apparatus the observer 
can stand ona platform of proper height, level 
or inclined, or he may sit ona high chair. In 
any case his eyes must be properly screened* 
so that he cannot see any outside objects. 
The apparatus is furnished with a large 
screened. metal screen not shown in the illustration. 
TABLE III. 
Mean of horizontal settings made while lying on the Mean of vertical settings made while lying on the 
Observer. left side, Tight side, left side, Tight side, 
by the left | by therig bythe = by the right | by the left | by the right by the left by the right 
eye. eye. eye. eye. eye. eye. eye. eye. 
1 —16° ——— + 9° —14° ———— +20° 
4 —10 —11° +10° + 6 — 6 —13° + 7° +4 
5 —11 +15 —7 —— —: +18 
8 —22 —13 +11 +17 —22 —15 +19 +14 
The observers were those denoted by the 
same numbers in Table I. 
The second piece of apparatus is repre- 
* In some experiments made recently in the labo 
tatory of Professor Cattell, of Columbia University, 
the observer was wisely stationed in a perfectly dark 
room. 
