indie ¢ ? A 
THE MICROSCOPE. : 43 
Fig. 3, Pollen of Ipomea Bona-nox. A clavate papilla. 
dy Se a a is Some of the intinal papillae 
withdrawn ; probably illusory. 
Fig. 5, Pollen of Ipomea coccinea; a single extinal papilla. 
Fig. 6, Pollen of Ipomea purpurea; reduced from W. G. Smith. 
BON tees Are Yt s <f a single extinal papilla. 
Ba ESO jot Cite ft rf be portion of a transverse — 
section. 
Fig. 9, Pollen of Ipomea Batatas; a single extinal papilla. 
Big. 10," - extinal papille, with villi. 
Fig. 11, Pollen of Ipomea pandurata. 
ds aap ara “ portion of a transverse 
section. 
cy i ke Sa of a single extinal papilla. 
Fig. 14, Pollen of Convolvulus Soldanella; after W. G. Smith. 
ees Lae gt Convolvulus arvensis. 
BO OS NY. org ‘ es a transverse optic sec- 
tion. Diagram. 
Fig. 17, Pollen of Convolvulus arvensis; after W. G. Smith. 
GLASS VERSUS METAL MICROMETERS. 
PROF. M. D. EWELL. 
THINK most persons who use stage micrometers in the ordinary 
way, prefer to have them covered, on account of there being less 
danger of injury and their always being ready for use: When my 
experience was less than it is now, I remember attempting to clean 
a really excellent micrometer by Prof. Rogers, one centimeter long 
ruled the whole length to ;,';, of a millimeter. I found out that it 
was uncovered after I had scoured the lines vigorously. It was then 
clean, but that was its only remaining recommendation. 
Professor Rogers has experimented much to avoid the sweating. 
that so often obscures the lines when the cover-glass is secured in 
' place by any kind of cement. The most successful method, I think, 
has been to rule the scale on a cover-glass and mount it with the 
lines downward, upon a thick ring perforated so as to allow a free 
circulation of air. This, again, has its peculiar disadvantages, as I 
have learned after the point of my objective (a ;'; Spencer) had 
gone through the cover. The lesson was more tmpressive after I 
had paid Mr. Spencer’s bill for re-centering the front lens. Micro- 
meters so mounted are very fragile, unless the cover-glass is too 
