ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 255 



about f in. to about <S in. in diameter. The metal band should not be 

 too stiff. 



The form of the tightening clip will depend on the use to which the 

 tank is to be put. If a clear opening be not necessary, the ends of the 

 band are turned up at right-angles, and a small bolt passed through 

 them (fig. 51), but if a clear opening be desired, the form shown in 

 fig. 52 should be used. 



Rock Crystal.* — F. J. Cheshire describes the geological conditions 

 of the places where rock crystal, the brazilian pebble of the optician, 

 is obtained, and gives an account of its crystalline nature. In con- 

 nection therewith, he points out that for high-power spectacle lenses 

 the crystal should be axis-cut, so that the effect of the double refraction 

 of the crystal is minimised as far as possible. 



Photogrammetric Focimetry.| — V. Legros treats this subject in a 

 series of articles whose nature may be gathered from his following 

 divisions of the subject : — 



Part I. Chap. 1. Principles of the method. 



„ 2. Errors of the method. 



„ 3. Determination of the nodal points. 



„ 4. Curvature of field. 



„ 5. Astigmatism. 



„ 6. Anomalies of focal length. 



,, 7. Definition, focal length, focal volume, luminosity. 



„ 8. Chemical focus. 



Part II. Chap. 1 . Relations of microbiology and of military technology. 



„ 2. Improvised microscopic focimetry. 



„ 8. Photogrammetric focimeter for microscopical optics. 



„ 4. Conclusions. 



A New Spherometer.J — This instrument for measuring the curvature 

 of lenses is described by C. V. Raper. The material for the framework 

 was made of Dr. Guilleaume's " Invar." In figs. 53 and 54 a sectional 

 elevation and plan are given, and it will be seen that the instrument 

 consists essentially of a tripod frame, and a very fine worm and worm- 

 wheel. The frame is built up of the invar rod-stays B and B x attached 

 to the top-centre A x . The two B stays have the conical-pointed feet 

 F affixed at their lower extremities, as plainly shown by the elevation. 

 The Bj stay, however, lying in the same vertical plane as the hori- 

 zontal lifting-bar H, is affixed thereto, and the B stays are similarly 

 attached to other horizontal stays H 2 (fig. 55). The horizontal lifting- 

 bars are screwed into the lower centre-piece A 1? both these (top and 

 bottom) centre-pieces being of invar. The invar tube C forming the 

 vertical strut, and also the bearing and nut for the worm-wheel spindle, 

 is a drive-fit into both centre-pieces, and is further secured in the 

 lower centre-piece A by the screwed ends of the two horizontal lifting- 



* Revue des Sciences Photographiques (Paris, 1 304), Nos. 1-8. about 72 pp., 3 figs. 

 + Brit. Optical Journ., 1904," pp. 202, 221, 239, 2C2 (20 figs.). 

 X English Mechanic, lxxx. (1904) pp. 358-00 (4 figs.). 



