ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 631 



(4) Photomicrograpliy. 



Photographic Foucault-pendulum."— E. F. Pigott has designed a 

 photographic arrangement for supplementing the visual method of 

 observation usually adopted in connexion with Foucault's well-known 

 pendulum for demonstrating the diurnal rotation of the earth on its axis. 

 He uses a large hollow heavy bob fitted internally with a small glow- 

 lamp and a short focus lens. The gyrations of the light-spot are recorded 

 on a sheet of bromide-paper placed below, and a permanent record free 

 from personal equation is thus obtained. 



Measuring the Focal Length of a Photographic Lens.f — The 

 principal focus of a lens of focal length / is at a distance /F//' from that 

 of the combination of focal length F formed by placing in front of the 

 first lens another of focal length /'. T. Smith points out that this 

 suggests a simple method of finding the focal length of a photographic 

 lens, which can be divided into two parts, each capable of producing 

 a real image of a distant object. Let / and /' be the focal lengths 

 of the two components, and F that of the complete lens. Set up the 

 whole lens in the camera and focus a distant object sharply on the 

 ground-glass. Now unscrew the front component of the lens from its 

 mount without disturbing the rest of the lens, and measure the distance 

 d through which the ground-glass has to be moved for the same 

 object to be sharply focused by the back component used alone. Then 

 d =fY/f'. Next take the whole lens out of the camera and reverse it, 

 so that what is usually the back component is now in front. Focus as 

 before with the complete lens for a distant object, and measure the dis- 

 placement of the ground-glass necessary to focus the same object when 

 the component now in front is removed. Denote this by d', then 

 d'=f'F/f; and, therefore, F- = dd. This method avoids the difficulty 

 of measuring exactly a transverse magnification, and also is not subject 

 to errors arising from want of parallelism of object and image, from 

 distortion and other oblique aberrations. The author gives several 

 developments of his method. 



(5) Microscopical Optics and Manipulation. 



Tracing Rays through an Optical System. | — The most trouble- 

 some calculations which have to be made in computing an optical system 

 are those relating to rays not lying in an axial plane. The methods 

 hitherto used are trigonometrical, and the formulae most extensively 

 employed are those of Von Seidel ; but T. Smith points out that, 

 although these equations have the advantage of being in a form suitable 

 for logarithmic computation, the process is very tedious — nine equations 

 have to be solved for each surface — and the method does not readily 

 indicate what modifications should be made in the system when the ray 

 does not emerge as is desired. An algebraic method is preferable, pre- 



* Journ. and Proc. New South Wales, 1. (1916) pp. 262-9 (1 pi.). 



t National Phys. Lab., Collected Researches, xiii. (1916) pp. 167-8; and Proc. 

 Phys. Soc. Lond., xxvii. pt. 2 (1915). 



X National Phys. Lab., Collected Researches, xiii. (1916) pp. 171-7 ; and Proc. 

 Phys. Soc. Lond., xxvii. pt. 5 (1915). 



