378 M. C. Lea on some new Manipulations. 
A slip of wood is provided 5 inch thick, 14 inches wide, and 
- 2feetlong. A piece of card about 4 inches square has two par- 
allel slits made in it at one end, one slit half an inch from the 
top, the other the same distance from the bottom. These slits 
are of such a length as just to permit the piece of wood to be 
passed through them, presenting the appearance represented in 
the margin. 
To make a reading, the half black card previously spoken of is 
slipped over the burette, and the line of separation between the 
white and black is brought one millimeter below the black line 
which marks the surface of the liquid. The instrument repre- 
sented in the margin, is then placed beside the burette, 5, 
the lower end resting on the table, and the card is raised 
or lowered until the edge B exactly corresponds with vi 
the line of separation in the black and white card. The 
stick is then drawn toward the observer, the end B ns 
still resting on the table, and the observer places his eye 
so as to keep the line B in range with the line of sep- 
aration, and makes the reading. 
Another precaution which I regard as essential, and 
which I have nowhere seen mentioned, is the follow- 
ing: The observer should place himself where he has 
a strong side light. The half black half white card must 
not be placed parallel to the eyes of the observer, but 
must be turned toward the light, so as to make an an-° 
gle of 45° with the line of vision. In this way a strong light 
is reflected from the card and thrown through the burette in a 
manner greatly conducive to clearness of vision, and the black 
line which marks the surface acquires a peculiar sharpness. — 
I cannot better illustrate the necessity of these precautions, 
(especially the use of the little instrament above described, and 
which may be termed the eye adjuster) than by the comparison 
of the following results obtained without them and with them. 
illed water corresponding to the divisions of a tube pre- 
pared in the manner above described was carefully weighed in 
an accurate balance. 
1. Without the precautions, 
~ Mean of first three trials, 2023 grams. 
“ 66 second “ “ 1974 “ 
2. With the precautions. 
Mean of first three trials,  2°046 grams. 
Tt eeoond *  S 2047 “ 
of = tae more <<)“ 2050 “ 
It will be seen that the trials without the precautions which I 
recommended gave results which were not only discordant with 
each other, but were all wrong and below the truth. With the 
