ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
225 
inventor’s object was to compare the number of nerve-cells in 0 * 001 cubic 
mm. of brain of normal and weak-minded individuals. 
Hammarberg’s method was to dispense with ocular micrometers, and 
instead to use the positive image of a 1 square cm. glass plate divided 
into 0 • 25 square mm., the image being thrown by a convex lens on to 
the preparation. The advantage of the arrangement is that, by increas- 
ing or lessening the distance of the lens, the image of the graduated 
plate on the preparation can be enlarged or reduced, and that this pic- 
ture becomes an integral part of the preparation ; magnification, there- 
fore, by the objective and ocular does not alter the ratio between the 
preparation and the scale. The glass plate is so arranged that each 
little square on the object is exactly 0*01 sq.mm. An inconvenience 
arising from spherical aberration in the peripheral parts is avoided by 
selecting a central portion. After counting the cells in a certain number 
of squares, those in ten consecutive serial sections of 10 /x thickness, or 
in five of 20 /x, are counted. This total is divided by the proper number 
of squares, and the quotient gives the number in 0*001 cubic mm. of 
brain cortex. 
Dr. Berger, having found the method satisfactory in brain investiga- 
tions, describes the details of the apparatus, and thinks that the idea is 
capable of further microscopic application. 
This ghost-micrometer was invented by Dr. C. R. Goring,* and was 
reinvented by Dr. G. W. Royston-Pigott,} and again by Prof. A. E. 
Wright.} 
Reichert’s New Form of Drawing Apparatus. — Fig. 57 shows this 
apparatus, which can be used with dissecting Microscopes and poorly 
lighted opaque objects as well as for ordinary work. 
Fig. 57. 
(4) Photomicrography. 
Culture Dish-holder for Microscopic and Photomicrographic Pur- 
poses^ — This has been made by' the Zeiss firm to Dr. W. Gebhardt’s 
* Micrographia, 1837, p. 51. f Monthly Micr. Journ., 1873, pp. 2 and 51. 
t Cf this Journal, 1897, p. 245. 
§ Zeitschr. f. wiss. Mikr., xiv. (1898) pp. 155-9 (1 fig.). 
