MJourual, Oct, 10. NOTES AND MEMORANDA. 235 
makers. The focal length of lenses of the same denomination is sub- 
ject to so great a variation that comparison of these by means of their 
assumed focal lengths too often gives no true idea of their relative 
excellence. For example, if two quarter-inch objectives be compared, 
and one gives results much superior to that given by the other, we 
cannot be at all sure that the better lens is not really of shorter focus 
than its designation would indicate.” He presents a table giving “the 
results of several hundred measurements on various objectives, and 
suggests that an examination of the table will show that the focal 
length of the objectives of some makers differs considerably from the 
length marked upon them. For example, No. 34 marked } inch is 
really a ird inch objective; No. 33 marked 3th inch is really a 1th 
inch ; No. 29 marked ;4,ths inch is really a 3th. Lens No. 14 marked 
ith inch is really a }th inch; but Nos. 13, 15, by the same makers, 
are correctly designated }th inch, 3rds inch. Differences of this kind 
must of necessity lead to a great confusion in comparing objectives 
with one another. I would therefore suggest that each objective made 
should be measured before being offered for sale, that this confusion 
may cease to exist. A convenient arrangement would be to fix a glass 
scale divided to =4,th or ;},th inch in the draw-tube, sliding in the 
tube of the microscope, and measure as I have already described. 
The draw-tube should be moved till the front of the ruled glass shall 
be exactly 10 inches from the micrometer used as the object. Or it 
would be more convenient still to have an apparatus similar to the 
first form, but arranged with a suitable stage and stand so that it can 
be set at any desired angle. The distance 10 inches (254 mm.), sug- 
gested as a standard, is chosen because it is the normal distance of 
distinct vision, as well as about the length used by microscopists in 
actual work.” 
Peedogenesis in the Stylopide.—Professor Von Siebold has dis- 
covered, says the ‘ American Naturalist,’ September, that the so-called 
female of Xenos is in reality a larva, and that it produces its young 
by germ balls like those of the larva of Cecidomyia (Miastor), which 
produces larve like itself during the winter months, but in summer 
undergoes the usual transformations of these gall flies. This child- 
reproduction, in individuals without true ovaries, was aptly termed by 
Von Baer “ Peedogenesis.” 
Air-tight Specimens.—A writer, who simply signs himself “R. 
H. W..,” writes to the ‘ American Naturalist’ to inquire when shall we 
cease to suffer from the directions sometimes given to mount dry speci- 
mens in a cell of pasteboard or paper, fastening the glass cover down 
by “a little gum” or “paste”? Of course dust or moisture soon accu- 
mulates in the cells, or fungoid vegetation grows until it becomes a 
beautiful and conspicuous specimen; but in any case the original 
object is tolerably certain to be marred or ruined. He has not unfre- 
quently seen collections of specimens, by popular makers, which have 
perished in this manner. Lately he lost in this way a very choice 
specimen prepared by one of the best Huropean makers, whose work 
is usually faultless; and still later, having occasion to remount a 
