ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 601 



constructed out of a Huyghens' ocular. For this purpose a glass micro- 

 meter is to be placed on the diaphragm, the divided face being under- 

 neath. A selenite or quartz wedge is attached to the micrometer with 

 Canada balsam. This wedge, which affords an easy means of obtaining 

 the colours of higher order, is compensated by a selenite plate of such a 

 kind that the iron-grey of the first order, the zero value of the inter- 

 ference colours, overlaps an initial point of the micrometer scale. It is 

 necessary to take care that the wedge has a uniform constant inclination 

 angle of 1° to 2°, and that in the combination of the two selenite plates 

 the axes of elasticity are accurately perpendicular to one another : thus 

 a x of the wedge is parallel to c 2 of the plate, and e x of the wedge is 

 parallel to a. 2 of the plate. The wedge must be so cemented to the 

 glass micrometer that the interference bands are parallel to the scale 

 divisions. The whole is covered with a circular cover-glass of such a 

 size that it will go into the ocular. The test is made with sodium 

 light, and the operator reads the number of micrometer divisions from 

 one dark band to the next. The wave-length (0 ■ 000575 mm. for Na) 

 is divided by the number of divisions observed. This calculation gives 

 the value of the phase-difference for one division of the scale. In order 

 to measure the double refraction of a given body placed on the object- 

 stage, an ocular of suitable strength, equipped as already described, is 

 inserted in the ordinary way in the tube and the analyser applied over 

 it. The object is then pushed along the scale until it has compensated 

 any particular position of the interference bands. It is necessary then 

 to bring the object into just such a position that the axes of elasticity 

 form 45° with the Nicol sections, which is easily controlled by the 

 adjustment of the micrometer scale, and then to rotate through 90° so 

 that the a axis of the selenite wedge lies over the axis of less elasticity 

 (c) of the object. The position, as found for compensation, is read off 

 on the divisions of the micrometer scale, and the number of divisions 

 (from zero of the interference colours) is then multiplied by the phase- 

 difference for one division. It is possible to compensate approximately 

 with ordinary light and then accurately for monochromatic. The author 

 gives some numerical examples. 



(3) Illuminating and other Apparatus. 



New Projection Drawing Board.* — A. C. Pohlman's arrangement 

 consists of an upright frame surmounted by three axles, each carrying a 

 reel of paper. The gearing is such that one or all of the reels may be 

 in action, and a crank in the lower part of the machine actuates a roller 

 for rolling up the paper wound off the reels. The paper in its descent 

 lies flat against a vertical board and receives the projected image. Means 

 are provided for keeping the paper taut. A brass bar acts like a knife 

 edge, and thus single drawings can be removed. If a series is required 

 the lower roller winds up the drawings. There is an arrangement by 

 means of which two sheets of double-sided blue copying paper can * be 



* Zeitschr. wiss. Mikrosk., xxiii. (1906) pp. 41-44 (3 figs.). 

 Oct. 17th, 1906 2 R 



