ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 677 



image of the object-centre comes in the centre of the screen. The fine 

 adjustment of the screen image is effected by the raising and lowering 

 of the object. In this way the object is brought into the intersection 

 of the optic axis of the camera and of the inclination axis. The illu- 

 mination, especially with reflected light, is of the highest importance 

 and should, as far as possible, fall perpendicularly on the inclination 

 plane ; otherwise, the two stereograms would be unequally illuminated. 

 The apparatus is made by R. Fuess & Co. 



New Method of Focussing in Photomicrography.* — Katharine Foot 

 and Ella C. Strobell add some notes on their method of focussing.f This 

 method offers special advantages for the vertical camera and daylight 

 illumination, as it does away with the use of the ground glass, a minus 

 spherical lens being substituted for the purpose of focussing. These 

 lenses can be obtained from any optician, and a series (omitting the half 

 numbers) ranging from — 1 D to — 12 D, will furnish the equipment 

 necessary for photographing at 1200 diameters or less, with most com- 

 binations of objective, eye-piece, and bellows drawn. The lens for a 

 definite magnification depends upon the eyesight of the operator. The 

 selection of this lens is a simple matter and can be determined by taking 

 one photograph. The method, in brief, is as follows : — " Instead of 

 attempting to focus on the ground glass fine details impossible to see 

 with daylight illumination, the change of focus necessary to throw the 

 exact image (selected for the photograph) on the ground glass, is accom- 

 plished by focussing through a minus spherical lens placed on top of 

 the projection ocular. This lens is removed before the plate is exposed. 

 The photograph is not taken through the lens. The use of these lenses 

 is simply a device for compelling the eye to see the plane of the prepara- 

 tion that is projected on the ground glass." Before exposing the plate 

 a delay of a few minutes is necessary to see that the focus does not slip. 

 It is also necessary to see that such a length of draw-tube is used as will 

 give agreement in results as tested by the Zeiss stage micrometer and by 

 the Zeiss micrometer eye-piece. A few photographs of the stage micro- 

 meter, taken with different combinations of lenses and bellows draw, 

 provide an accurate register of magnifications, in convenient form for 

 reference in selecting the lenses and draws needed for a given magnifi- 

 cation. In using this method of focussing, it is a great aid to determine 

 the limits within which a sharp focus can be expected, for it is easy to 

 strain the eye and see details beyond these limits ; the negative in this 

 case giving disappointing results. 



Photographic Lenses. J — Under this title, C. Beck and H. Andrews 

 have compiled a book intended for the use of the non-mathematical 

 photographer. But so much of the work is occupied with an explanation 

 of the properties of lenses that it cannot fail to be of interest to micro- 

 scopists. The diagrams and illustrations are very numerous, and the 

 plates devoted to such subjects as curvature and distortion are remark- 

 ably effective. 



* Journ. App. Micr., v. (1902) pp. 2082-4 (1 fig.). 



t Zeitschr. wiss. Mikr., xviii. (1902) pp. 421-6 (1 pi.) ; and this Journal, 1902, 

 pp. 490-1. 



% Published by R. & J. Beck and Percy Lund, Humphries & Co. (second edition) 

 London. 



