ZOOLOGY AND BOTANY, MICItOSCOPY, ETC. 103 



most convenient position of the object. When it is required to produce 

 photographs by the objective alone, a special tube J is provided, 75 cm. 

 long, open at its upper end, but carrying at its lower end a diaphragm 

 of 25 mm. diameter. The tube is controlled by push-movement, and 

 can be manipulated until a perfect image is obtained. The object-stage 

 is 12*5 by 15 cm., and has three grooves at its narrow sides for various 

 exchangeable accessories. A blackened metal plate R, with object- 

 clamps, inserted into the uppermost groove, forms the object-stage 

 proper. The second groove is for an opal disk to secure uniformity of 

 illumination. The third groove is for obtaining a dark background, 

 the mirror S being removed and the wooden box Q (blackened inside) put 

 in its place. In the case of larger objects, dark-ground illumination is 

 secured by removal of the box and by placing the object on black card- 

 board. For transparent objects on a bright ground, the mirror itself 

 serves as an object-stage, and is placed in the uppermost groove. For 

 opaque objects on a bright ground, a strong illumination is directed 

 from above on to the object, whilst the mirror (now an opal glass plate) 

 is illuminated from a weaker source. This method has the advantage 

 of almost eliminating the shadow. 



(5) Microscopical Optics and Manipulation. 



Measurement of Resolution in Microscopy.* — C. Fabre discusses 

 the theory of microscopical resolution, and emphasises the results of his 

 experiments with Grayson's test-plates. He has found plate No. 6, de- 

 signed for use with objectives of large aperture, especially satisfactory. 

 On this plate the lines of the first group are at intervals of 10,000 to 

 the inch ; those of the next group contain double that number ; and in 

 the last group there are 120,000 to the inch. A prolonged use con- 

 vinced the author that this plate is the best means of measuring the 

 resolving power and the defining power of an objective. The length 

 and the regularity of the lines give also a very clear notion of the 

 curvature of the field of the objective under examination. The author 

 also points out that knowledge of the resolving power of a lens may 

 prevent false decisions as to the existence, or otherwise, of micro- 

 organisms in an object. 



New Method of Measuring Directly the Double-refraction in 



Strained Glass.j — L. N. G. Filon describes his method for the above. 



A horizontal beam of parallel homogeneous light is made to impinge 



normally on a vertical face of a rectangular horizontally-placed glass 



slab, subject to vertical flexure. If C x = stress-optical coefficient for 



the ray polarised in the plane of the cross-section, and for light of the 



given wave-length ; M = bending moment ; I = moment of inertia of 



the cross-section about the " neutral axis " ; and T = thickness of the 



slab, then the points at which the disturbance is in the same phase can 



be shown to lie upon a straight line inclined at 0, to the vertical, where 



O MT 

 #i = --y — . Such a slab under flexure will deflect the wave-front like 



* Mem. Acad. Sci. Toulouse, vi. (1906) pp. 142-9. 



t Proc. Roy. Soc, Series A, lxxix. (1907) pp. 440-2 (1 fig.). 



