230 Relation of Devonian Faunas of Iowa. — Williams. 
used for mounting which will avoid any interference with the revolu- 
tion of the stage. 
Into the nose-piece, just above the objective, is an opening intended 
to receive the four following accessories, each mounted in a separate 
brass frame: (1) a Bertrand lens for magnifying the interference 
figures ; (2) a quarter-undulation mica-plate ; (3) a quartz wedge ; 
(4) a Klein quartz-plate or a gypsum plate with red of the first order. 
The centering of the various objectives is secured by two screws hav- 
ing motions at right angles to each other. 
The upper Nicol-prism or analyzer is inserted in the tube in order to 
avoid the diminishing of the size of the field which is unavoidable 
when the prism is placed over the ocular as a cap. To accomplish 
this and at the same time to keep the tube dust-tight the Nicol is 
enclosed in one side of a double chambered box. The other side is 
left vacant and the box may be slid to and fro according as ordinary or 
polarized light is desired. A metal sheath protects this box from 
above. 
The microscope, as here described, in a case with a single eye-piece 
but without objectives, may be obtained for .$108.00. With two eye- 
pieces (one with cross hairs and the other with micrometer) and two 
objectives {% and 1 - 5 inch) its cost is $1.35.00. The cost of a solid brass 
stand is about $25 00 more. 
The demand in the United States for microscopes adapted 
for the examination of minerals in thin sections is rapidly 
extending. This is due to the, hitherto, small amount of 
attention that has been given to this science in America and 
the awakening that has marked the past ten years. It is also 
due, in a large measure, to the prevalence of the Archaean 
rocks in those regions of the country where the advanced 
schools of science are found. This brings the science of micro- 
scopic petrography prominently before both the student and 
the teaching geologist, and they find naaterial ready at hand 
with which to work. It remains yet to see a first-class petro- 
graphic microscope of American manufacture. Those above 
described are adapted to ordinary use and the necessities of 
the instructional laboratory. They do not embrace the ex- 
tended apparatus for nice distinctions, and for elaborate re- 
search, seen in some of those of European make, like that of 
the Grand Models of Nachet. 
ON THE RELATION OF THE DEVONIAN FAUNAS OF IOWA. 
By H. S. Williams. 
The two papers of professor Calvin, which were published in 
the Bulletin from the Laboratories of Natural History of 
the State University of Iowa, November, 1888, Vol. 1, No. I,' 
' Some geological problems in Muscatine County, Iowa. With 
apecial reference to the rectification of the supposed Kinderhook near 
