ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 325 



dirticulties experienced in focusing a hiu-li-power objective. A small 

 universally adjustable mirror in a circular frame is clamped on to the 

 object-strige, and so orientated that it reflects object and objective to 

 the eje in just such a manner as the eye would see them if it were 

 itself in the plane of the object-stage, and at a suitable distance. The 

 operator can therefore apply himself with confidence to the work of 

 focusing, with occasional glances at the mirror. The distance between 

 objective and object is made much more distinct if a slip of celluloid is 

 inserted behind the objective. The separating air-space then appears as 

 a fine white Une. 



Dark-ground Illumination.* — " Queketter " states that an effec- 

 tive dark-ground illuminator may be made by placing between the 

 top and next lens of the Abbe illuminator, an opaque disk of such a 

 diameter as will cut off all the rays that directly enter the objective. 

 The simplest way is to take some tinfoil and make a disk within a frac- 

 tion of the diameter of the upper side of the second lens — that is, the 

 lens immediately behind the front lens — of the condenser. The disk is 

 then rested on the upper side of the second lens and made to stay in 

 position by means of a little immersion oil or similar material. The top 

 lens is then screwed on and placed in immersion contact with the under- 

 side of the slide. Xow if a ^ or ^ objective be used on the object 

 (which must be mounted in a medium other than air), it will soon be 

 seen whether the object or particles are lit up on a black background. 

 If no light passes, reduce the size of the tinfoil disk and repeat the 

 experiment until the desired effect is obtained. This device gives a 

 result equal to that obtainable with the expensive immersion dark-ground 

 illuminators. 



(5) Microscopical Optics and Manipulation. 



Application of Optical Methods to Technical Problems of Stress 

 Distribution. f — E. G. Coker describes the adv-mtages as regards 

 cheapness and transparency of nitro-cellulose compounds for the manu- 

 facture of models of machinery and mechanical structures. When 

 subjected to loads, strains are 'set up in the material which, under 

 crossed nicols, reveal themselves as colour fringes. Fig. 46 shows 

 such an example, the coloured parts appearing black in the photograph. 

 The difficulty and expense of procuring large-size nicols, has, however, 

 led to a method of obtaining polarized light on a large scale, and this is 

 applicable to the examination of models of objects such as girders, ships, 

 and bridge-structure. Fig. 47 shows an apparatus of this kind due to 

 Silvanus P. Thompson. Light from a bank of lamps A, is diffused by 

 tissue paper screens B, and afterwards reflected from a black glass plate 

 C set at the polarizing angle. Quarter- wave plates D and E are 

 arranged to produce a circularly polarized field in the object-space F, 

 and for demonstration purposes the analyser is constructed of thin glass 

 plates G, while a small nicol prism is used for quantitative work. This 

 apparatus is capable of affording a clear field of view through quarter- 



* Knowledge, xxxvi. (1913) p. 148. 



t Nature^ Ko. 2249 (Dec. 5, 1912, pp. 383-6 (6 figs.). 



June IS, 1913 z 



