Ea 
. 448 ` NOTES. 
below so far that the glass disc is considerably within the working 
focus of the lens: Thus protected the lens can be lowered into 
water, syrup, glycerine, etc., to a depth limited only by the mech- 
anism of the microscope or the length of the protecting tube, and 
used to view objects floating in the liquid or lying on the bottom 
of the vessel containing it. While the common ‘tank micro- 
scope” can be worked best somewhat horizontally, through the 
side of the tank, this arrangement, besides being applicable to 
much higher powers, is adapted to give a more or less vertical 
view, being entirely free from any tremor on account of the motion 
of the top of the water, and is therefore especially useful for dis- 
secting purposes. Its object, though not its method, is identical 
with that of Tolles’ immersion objective for low powers, published 
more than two years ago; though the latter naturally possesses, 
being constructed especially for this use and dispensing with two 
unnecessary surfaces of glass, some optical superiority as well as 
a much longer working focus. The submersion tube, being appli- 
cable to ordinary lenses, only slightly lowering their magnifying 
power and considerably shortening their working focus, will doubt- 
less be extensively useful; though the statement that it may be 
always retained in position as a protecting cover to the lens with- 
out impairing the definition or illumination in ordinary work, must 
be considered as too enthusiastic. It is especially applicable to 
lenses of from one inch to one-quarter inch focus (the latter lim- 
ited to a very small angle), and the objects should be placed ina . 
jar or tank having the bottom and at least one side quite smooth 
and transparent.—R. H. W. 
cae is 
—_— 
NOTES. 
AERA 
Prof. C. F. Hartt, who has already become identified with Bra- 
zilian geology, zoology, and archæology, from his three former 
expeditions, in the first being an assistant to Prof. Agassiz on the 
Thayer expedition, left New York on July 24th for another ‘and 
more extensive exploration of the eastern part of the valley of 
the Amazons. He will continue the series of geographical and 
geological investigations begun on his last journey, and expects 0n 
his return to be able to publish a detailed account of the region 
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