ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



701 



(fig. 92), 25 mm. long and 15 mm. broad, are set in a rectangular frame 

 at a distance of 6 cm. apart. They are movable about the axes A B and 

 C D and can be fixed in any required position by means of the screws A 

 and C. The frame is fastened to a staud adjustable in height, and is 

 movable about the axis E F. The mirror A B has a portion in the 

 middle jjlain. For determining the magnifying power of a Microscope 

 the frame is set horizontally, and both mirrors are fixed at 45^ to the 

 horizontal with A B directly above the eye-piece, and C D above the 

 object intended for comparison which lies by the side of the Microscope 

 and is seen by double reflection at the same time as the image of the 



Fig. 93. 



nr-^ 



Fig. 94. 



object beneath the Microscope. For the latter the author uses a 

 micrometer scale with lines at distances of 1/10 mm. and 1/100 mm., 

 and for the comparison object an isosceles triangle, with base 20 mm. 

 and height 100 mm., printed on grey j)aper. It is easy to see how many 

 micrometer divisions corresj)ond to one of the triangle, thus in fig. 93, 

 4 mm. divisions fall on the No. 8 division which would correspond to a 

 magnification of 20 times where the micrometer divisions are tenths. 

 The proper magnifying power is then this number multiplied by the 

 ratio of the distance (a + b) of the object of comparison from the eye to 

 25 cm. (least distance of distinct vision). (Cf. fig. 94.) 



Bbadt, N. — Illustrations of Diffraction. 



[" My purpose this evening is to show by actual experiment how even a simply 

 constructed Microscope may be made a most valunble instrument in ex- 

 amining the phenomena of this branch of Physical Optics, and to illustrate 

 how cheaply and how easily many interesting diffraction experiments may 

 be made."] 



Paper read before the Western Microscopical Club, March 4th, 1889, 10 pp. 

 Light ox, W. — Instantaneous Changes of Field. 



[Instantaneous changes from dark field to light field and back again with the 

 largest numerical aperture possible.] 



A))ier. Mon. Micr. Journ., X, (1889) p. 164. 

 Nelson, E. M. —Diatom Structure. 



Trans. Middlesex Nat. Eist. Soc, 1889, 13 pp. and 1 pi. of photomicrographs. 



