INVERTEBBATA, CRYPTOaAMIA, MICROSCOPY, ETC. 331 



phragms of this instrument.* As at first made, it was provided with 

 a diaphragm for using one or two oblique pencils only. The neces- 

 sary movement was made by means of a small projecting arm A 

 (Fig. 18), which was inconveniently near the stage. 



The diaphragms (l^'igs. 19 and 20) now have a central aperture, 

 and the movement is made by means of an outer sliding tube b with 

 a slot at the top, in which the arm A fits, and another arm B is 

 placed at the lower end so as to give ready command of the rota- 

 tion. The new plan allows of the use of either central light alone, 

 or one or two oblique pencils incident 90° apart in azimuth. D is 

 the optical part of the condenser, placed immediately above tho 

 diaphragms, and in oil-immersion contact with the base of the slide. 

 The circular diaphragm is fixed into the inner tube attached to the 

 substage tube C, just below the position of the arm A ; and the other 

 diaphragm is screwed to it by a screw in the escentrical hole, shown 

 in each. It will be seen that when the diaphragms are placed 

 together in this manner, the movement of the arm will produce tho 

 changes in the light as above mentioned. 



Illumination with High Powers. — At the March meeting, Mr. 

 James Smith explained a simple and effective method which had 

 occurred to him of illuminating Podura scales, diatoms, &c., under 

 powers as high as the j^^, by using the ordinary bull's-eye condenser 

 only. The mode in which Mr. Smith prefers to use the condenser is 

 to place the lens just above the stage with the flat side uppermost, a 

 lamp being in front at a distance of 2 or 3 inches. The light enters 

 the condenser, and is reflected from the plane surface at a very oblique 

 angle upon the slide. 



West's Universal-Motion Stage and Object-holder. t — Mr. E. 

 G. West proposes a new arrangement for examining those bodies 

 which, when seen obliquely, show some features of colour or structure 

 that are invisible when they are viewed perpendicularly to their sur- 

 faces. Morris's object-holder has the drawback that, with the excej)- 

 tion of rotation of the carrier in a horizontal position, every move- 

 ment involves a lateral and vertical displacement of the object, which 

 is thus continually thrown out of the field and out of focus. 



The principle of Mr. West's arrangement is that the object is 

 placed in the ventre of a movable sphere, so that all lateral displace- 

 ment is abolished, and the small inevitable focal displacement of tho 

 margin of the object is reduced to a minimum, while tho centre re- 

 mains undisturbed. 



I'igs. 21 and 22 represent the apparatus, the latter figure being a 

 transverse section of the former. A A is a base plate, with an aperture 

 Z in its centre. To this plate is soldered a sliort brass tube C C. 

 D D is tlie carrying plate (with a central aperture X) and to tho 

 lower side of which is soldered a brass hemispherical shell E E, which 

 has also a circular aperture V, on tho under side for about a third of 



* Sec thifi .Jinirnul, unlr, p. 1 17. 



t Mourn, (iiuk. IMirr. Cliil),' vi. (18S{)) p. 'if). 



