ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 
89 
adjustment, and by expressing his surprise that so little is met with on 
the subject in optical treatises. 
Faultless centering occurs when the thickest point of convex lenses 
(and the thinnest point of concave ones) lies in the centre of the circular 
periphery of the lens. Moreover, the line joining the two sphere-centres 
of the lens must he perpendicular to the two surfaces of the lens. A 
historical account of the means adopted by early opticians to attain 
these conditions is given. References are made to Prechtl’s ‘ Praktische 
Dioptrik’ (Wien, 1828), to Pobert Smith’s ‘Optics’ (1755), to Kliigel 
(Wien, 1765), and to Fraunhofer’s methods. It appears that Fraunhofer 
published nothing on centering, but the author has met with a tradi- 
tional account of his manner of working. The paper contains (p. 173) 
an account of the method of centering large flattish lenses adopted by 
Ross. 
(3) Illuminating- and other Apparatus. 
Behrens’ New Projection Apparatus.* — Prof. Behrens’ object was 
to produce an apparatus cheap, handy, portable, which should serve all 
but the highest purposes of demonstration. After discussing the advan- 
tages and disadvantages of the various luminants, he selects lime-liglit as 
the best for general use, but points out that the usual burner is faulty, 
and describes his design for improving it. 
Fig. 10. 
The general arrangement of the apparatus is shown in figs. 10 and 11. 
The only wooden parts are the mahogany base-board (67 X 26 cm.) aud 
the objective-board 0. The larger metal parts are of rolled aluminium, 
and the brass parts are nickelled. The camera C, internally lined with 
* Zeitsclir. f. wiss. Mikr., xv. (1898) pp. 7-2? (5 figs.). 
