ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
861 
statement about Canada balsam and its refractive index (which is against 
all that the best authorities have discovered, viz. that the refractive 
index of Canada balsam is />t D 1 * 540) involving a dispersion of nine times 
and a refraction of three times too much for optical balance when com- 
pared with castor-oil, yet the effect of a thin film with parallel surface 
cannot be sufficient to make any perceptible difference. 
Castor-oil, moreover, crystallizes, and is practically unsuitable for a 
permanent cementing medium. 
C3) Illuminating- and other Apparatus. 
Standard Glass and Speculum Metal Centimetres.* — Dr. M. D. 
Ewell writes: — “I have ruled two centimetres on speculum metal sub- 
divided into millimetres, the first millimetre into 1/10, and the first 
1/10 mm. into 1/100 mm. These were sent to Prof. William A. Rogers 
several months since for investigation as to total length. 
The five glass centimetres similarly subdivided, which were made 
the subject of a communication at the last meeting, have been in my 
possession the last three or four months, but are now in the possession 
of the Treasurer. My experience with glass micrometers has been such 
as to lead me to delay their investigation till sufficient time has elapsed 
to ensure their durability. 
The lines on the five centimetres above referred to are still in as 
good condition as when made, and bid fair to be permanent. 
The lines on the Fasoldt centimetre No. 2 (glass), upon which so 
much time was spent some years since by myself in comparing it with 
‘ Centimetre A,’ have so deteriorated as to make it entirely worthless 
for micrometric purposes, notwithstanding they are covered, and there- 
fore not exposed to the deteriorating influences operating upon ordinary 
glass micrometers. Owing to the uncertain life of such scales, it has 
been thought wise not to bestow so much labour upon the five glass 
centimetres above referred to till more time has elapsed. I have, how- 
ever, in order that they may be available for present use, compared the 
first 1/10 mm. of each of said scales with the first 1/10 mm. of ‘A,’ 
and deduced the following provisional corrections, which are the mean 
of ten measurements of each space. 
The comparisons, of which the following are the results, were made 
by means of a Bulloch Professional stand No. 2, a Zentmayer filar 
micrometer, and a Bausch and Lomb one-half opaque illuminator, with 
daylight received from the right of the instrument : — 
Scale, 
Correction to First 
Correction to First 
1/10 mm. 
1/5 mm. 
Ewell 
i. 
_ 
.. .. 0-5 
Not determined. 
ii. 
+ 
.. .. o-o 
+ 0*4. 
• • 
hi. 
— 
.. .. 0-4: 
■ — 
IV. 
+ 
.. .. 0-2 
Not determined. 
» • • 
v. 
. . 0 • 5 
Proc. Amer. Soc. Mier., xiii. (1891) pp. 71-2. 
