
REVIEWS. 101 
was doubtful. Another trial of the Hartnack No. 10 resolved the 13th 
band perfectly, —the 14th doubtfully. 
nglish and American opticians name their objectives (i. e., the lens or 
lenses placed next the object, that next the eye being the eye-piece), from 
their magnifying power,—thus a finch objective has the same power 
a simple lens of 4 inch focus. Continental European makers generally 
distinguish their instruments by numbers; the higher numbers indicating 
higher powers; but as each maker has his own system, the actual power 
of an instrument must be ascertained by trial. Instruments also often 
differ from their names, and they cannot generally be depended on. The 
theoretical power of a ARREA is measured from an arbitrary standard 
of ten inches,—thus, a one inch is said to magnify ten diameters; a 4 
inch, forty diameters. If aia a is taken at five inches, as it is by 
some, then the ‘‘ power” is but one half as much. The ‘‘ power” of the 
microscope is that of the bees multiplied by that of the eye-piece; if 
the objective "so ten diameters, and the eye-piece ten, the result is 
one a diameters. 
Angular ict is the angle in the surface of the front lens, at which 
light se enter the objective,—the greater ca angular aperture, the 
o 
An amplifier is an achromatic combination isori in the compound 
body of the instrument to increase the ‘‘power” of the objective and 
eye-piece 
iarston lenses have lately attracted great attention, though they 
were made by Amici many years since. e objective is immersed in 
water, —that is, there is a film of water between the front of the object- 
ive and the object, or the thin glass covering it. The effect is a great in- 
crease of light, and better definition. 

REVIEWS. 
ANIMAL NATURE OF SPONGES.*—Many opinions have been ex- 
‘aaa with regard to the animal nature of the sponge, which h has been 
considered as a plant by most authors, but nothing of a reliable or ge 
nite nature had appeared before a paper by Mr. Carter in the PERE 
Magazine of Natural History, for April, 1857. In this paper it w 
shown that the organized layer of the sponge was made up of pie 
eel 


*On the Spongiæ Ciliate as Infusoria Flagellata; or, Observations on the Structure, Ani- 
mality, and Relationship of Leucosolenia go granes Bowerbank. With two plates, and more 
than seventy-eight figures. By Professor H. James Clark, A. B., B.S. Memoirs of Boston 
Society of Natural History, Sad 20, 1866. 
