ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 751 



(2) Eye-pieces and Objectives. 



Direct Determination of the Curvature of Small Lenses.* — C. V. 

 Drysdale exhibited and described apparatus for the direct determination 

 of the curvatures of small lenses, such as the objectives of Microscopes. 

 Parallel light from a distant source falls upon a plane unsilvered mirror 

 inclined at an angle of 45°. Some of the light is reflected and brought 

 to a focus by an ordinary convex lens. The surface to be tested is 

 placed at this point, and the reflected rays proceed as if they had come 

 from a point on the surface. They pass through the plate glass into a 

 telescope focused for parallel rays, and an observer sees an image of the 

 distant source. If the surface is convex and is brought nearer to the 

 lens, then, when it reaches such a position that its centre of curvature is 

 at the focus of the rays emerging from the lens, the light will again 

 retrace its former path, and a distinct image of the source will be seen 

 in the telescope. In order to obtain the two images, the surface has 

 therefore been moved through a distance equal to its radius of curva- 

 ture. If the surface is concave, it must be moved away from the lens. 

 The author showed how the method could be carried out by means of 

 an auxiliary piece fitted to an ordinary Microscope. He also described 

 a method of testing the spherical and chromatic aberration of micro- 

 scopic objectives. Light from a distant point is partially reflected by 

 means of a piece of plate-glass down the axis of the Microscope. In 

 passing out of the objective it is brought to a focus upon a mirror, 

 and retraces its path along the axis of the instrument until it reaches 

 the plate glass. It passes through, and by means of a telescope an 

 observer can view the distant source. The light having passed twice 

 through the lens to be investigated, the effects of chromatic and spherical 

 aberration are doubled, and at the same time the effect of coma is 

 eliminated. 



(3) Illuminating- and other Apparatus. 



New Ultra-Violet Mercury Lamp (Uviol Lamp).f — 0. Schott and 

 those who work with him at problems involving ultra-violet rays have 

 found " Uviol " a convenient abbreviation. In the construction of this 

 lamp full advantage has been taken of that new Jena glass which is 

 pervious to ultra-violet rays. Platinum wires are fused into the 

 extremities of a suitably shaped, generally straight, uviol-transmitting 

 glass tube of from 8 to 30 mm. diameter, and of a length of from 20 to 

 130 cm. The platinum wires terminate inside the tube in the form of 

 carbon heads, and admit of the use of either pole as positive or nega- 

 tive. Interiorly the lamp requires a mercury charge of from 50 to 

 150 grm. according to its size. The purpose of the mercury is not 

 only to supply the vapour required for illumination, but also to effect 

 the starting and to divert heat in order to cool the negative pole. The 

 lamp is started by tilting ; the two poles then become connected by the 

 mercury, the current having, of course, been previously switched on. 

 At the first moment of contact between pole and mercury, part of the 



* Nature, lxxi. (1904) p. 142. 



t Schott and Ger.. Jena. Pamphlet No. 421, 10 pp.. 1 pi., 1 fig. ; Nature, 1873 

 (1905) p. 513. 



