ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
139 
My method of determining whether the curvature of the cover-glass 
sensibly affected micrometric measurements was carefully to measure tho 
same space under identical conditions, except as to tho cover-glass, and 
to compare the results. For this purpose a high-angled dry objective 
was employed, as being more likely to be affected by a variation in the 
cover-glass than a homogeneous immersion or low-angled objective. 
The cover-correction was made as perfect as possible and not changed 
thereafter. 
The measurements were all made with a Bausch & Lomb first-class 
1/6 objective (cover-correction = 0 — (— 3 — (— 1/2 -j- 2/10), a Bulloch filar 
micrometer, a Smith’s vertical illuminator, manufactured by Mr. W. H. 
Bulloch, and a Bulloch stand, ‘ Professional No. 2.’ 
The space measured was the second 1/100 mm. of ‘Centimeter A,’ 
counting from the first line of the 1/100 mm. band. The temporary 
covers, except as otherwise noted, were 16/100 mm. in thickness, and all 
except numbers 1, 7, and 9 were taken at random from covers purchased 
in the ordinary course of trade or furnished by the courtesy of the 
Bausch & Lomb Optical Company and the Palmer Slide Company. 
No. 1 was so nearly flat that its curvature, if any, could not be detected 
by the second telescopic test above described. Nos. 7 and 9 were covers 
ground and polished to order by Messrs. Bausch & Lomb, with the 
express purpose of making them as nearly flat as possible, but with the 
effect of making them more curved than covers usually are which have 
not been thus treated, experience showing that the covers are so easily 
warped that they cannot by ordinary methods be ground flat. 
The measurements are recorded in terms of divisions of the micro- 
meter of which 1 division = 0-0732 p,. The number at the head of each 
series refers to the temporary cover used in the series. 
October 6, 1889. 
No. 1 (fiat cover). 
131-4 div. 
131- 2 „ 
133-0 „ 
132- 0 „ 
133- 0 „ 
Mean: 132 1 
Mean: 131-8 
No. 3. 
130-0 div. 
3-3 „ 
1-9 „ 
0-7 „ 
1'8 „ 
Mean: 131 "5 „ 
No. 4. 
131-7 
3-9 
10 
2-9 
1-0 
div. 
Mean: 132-1 
No. 5. 
131-9 div. 
1-0 „ 
1-8 „ 
1-2 „ 
1-7 „ 
Mean: 131-5 
No. 6. 
131-7 div. 
1- 7 „ 
2 ' 5 „ 
2- 4 „ 
11 „ 
Mean: 131-9 
