ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 529 



author describes how a series of photomicrographs upon such a group 

 of mercury o;lobules would make it possible to ascertain the exact motion 

 of the trunnion axis and hence arrive at a suitable correction factor. 



Measurement of Magnifying Power.* — M. A. Ainslie suggests the 

 following simple methods for obtaining the magnifying power of a 

 Microscope. 



1. Projection on a Screen. — Excellent results can be obtained with a 

 paraffin lamp and a bullseye. The top lens of the condenser should 

 be removed, and, in order to sharpen the outline of the circle of illumina- 

 tion seen on the screen, the iris diaphragm of the condenser should be 

 closed as far as possible, consistent with the easy visibility of this circle. 

 The Microscope is placed horizontally and the objective removed. The 

 screen should be accurately at right angles to the axis of the Microscope 

 and 250 mm. from the Ramsden disk. If D be the distance of the 

 screen and B the diameter of the circle of light formed there, then C 



= . This quantity C is called by E. M. Nelson the " eye-piece 



constant," and is really the diameter of the projection of the circle 



formed by the diaphragm of the eye-piece on a plane 250 mm. from 



the eye. If now, an objective having been inserted, an object of 



diameter A just fills the field, M (the magnifying power of the objec- 



C 

 tive) = — . A would probably be obtained from a stage micrometer 



or a divided scale. 



2. Pin Method. — The Microscope being placed horizontally, a 

 board about a foot square is supported horizontally just below the 

 optical axis of the Microscope. A cross-line being drawn on this 

 250 mm. from the Ramsden disk and at right angles to the optical axis, 

 two pins are stuck into the board on this line in such a position that 

 they are in line with the Ramsden disk and the observer's eye at the 

 moments when the Ramsden disk disappears — which it will do when 

 the observer's eye, placed in rear of the board, is moved far enough to 

 either side of the optic axis. If care is taken that the cross-line i& 

 exactly 250 mm. from the Ramsden disk, this method gives C without 

 further calculation, by measuring the distance between the two pins. 

 In this method the Ramsden disk should be reduced to a mere point 

 by closing the sub-stage iris. For correct results, uniform and complete 

 illumination of the field of view is essential. 



8. Sextant Method. — This method is capable of very accurate 



results. If $ be the angular subtense of the field of view, as seen in 



6 Q 6 



the eye-piece, then tan - = — ^ 250, or C = 500 tan -. If a sextant, 



provided with a low-power inverting telescope with cross-wires in its 

 eye-piece, is supported horizontally in such a way that the large or 

 index mirror is close to the eye-piece, the edges of the field of view can 

 be brought successively, after reflection at the two mirrors, to coincide 

 with the vertical wire in the eye-piece of the telescope (previously 



* Eng. Mechanic, xcviii. (1913) pp. 12-13. 



Od. 15th, 1913 2 N 



