566 The American Naturalist. [June, 
New Method of Illuminating in Photomicrographic Work. 
—Kéhler" has suggested a method of securing even illumination of the 
field when artificial light is used. Instead of removing the condenser 
and collector from the microscope, as is usually done, thus securing an 
image of the source of light in the plane of the section, Köhler makes 
use of an accessory lens and so adjusts the condenser that a sharp 
image of the accessory lens is brought to the plane of the section. The 
object is thus uniformly illuminated, even to the margin. 
Chemical Behavior of Dimorphous Minerals.—Doelter” has 
studied the comparative action of reagents on some dimorphous min- 
erals, viz.: andalusite and kyanite, orthoclase and microcline, epidote 
and zoisite, enstatite and anthophyllite, diopside and actinolite, pyrite 
and marcasite, and sphalerite and wurtzite. Finely powdered speci- 
mens of each were subjected under similar conditions to the action of 
such reagents as chlorine and hydrochloric acid gases, hydrofluoric 
acid, potassium and sodium hydroxides, ete., to determine their rela- 
tive decomposability. Marcasite is found to be less decomposed by 
solution of soda than pyrite. The fact that on treatment with water or 
sulphide of soda, the mineral which separates from the solution on 
evaporation is always the particular modification which was dissolved, 
seems to show a chemical difference between the two dimorphous 
forms of ZnS and those of Fe S, In many other cases the results were 
negative or the differences were such as might be explained by the 
slight chemical differences of the substances taken. 
Pearls.—Though perhaps not strictly to be included in the field 
covered by these reviews, it seems proper to call the attention of mineralo- 
gists to the admirable paper by the late Professor Karl Möbius on 
pearls", in Velhagen and Klasing’s popular magazine. This scientific 
paper discusses not alone the methods of fishing and extracting pearls, 
but describes, with the aid of beautiful figures, the different fresh and 
salt water mussels which bear pearls, the structure of the animal, and 
the manner of growth of the pearl. The structures of the pearl itself 
are made clear by drawings from microscopic sections, prepared by the 
author from a number of valuable gem pearls. The connection be- 
tween the structure and surface and the value of the gem is also dis- 
cussed.— Wa. H. Hoses. 
u Zeitsch. f. Wiss. Mikrosko = p. 443 (1893). Abstracted in Zeitsch. f. 
Instrumentenkunde, 14, pp. 41 1(1 1894 94). 
Neues Jahrb. f. Miner., etc., om ip pp. 265-27 
te 
18 Die echten Perlen. Velliagen und Klasing’s Monatsh IXte Jahrgang, 
pp- 325-335. (Nov. 1894.) on 
