ZOOLOGY AND BOTANY^ MICEOSGOPY, ETC. 



523 



openings in the circular disk of apertui-es supplied by Powell and 

 Lealand with their illuminator. 



Stephenson's Immersion Illuminator. — Fig, 112 shows the general 

 appearance of this illuminator, of which a diagram only was given 

 ante, p. 208. There are two sets of diaphragms, one on a vertical 

 revolving disk, and the other on a horizontal sliding plate, intended 

 for the higher and lower apertures respectively. 



Fig. 112. 



Fig. 113. 



West's Adjustable Dark-ground Illuminator. — In the cabinet 

 of the late Mr. F. L. West, optician, of Cockspu^-street, W., we found 

 the dark-ground illuminating device shown in fig 113. 



It consists of a plano-convex lens through the axis of which a hole 

 is drilled to receive a socket with an internal sere v. In the socket is 

 a steel rod carrying a disk of thin metal, which lies over the plane 

 surface of the lens. According to the distance of the disk from the 

 lens more or less of the central rays are shut off. The rod is sup- 

 ported on a pivoted arm similar to that ordinarily used for Barker's 

 wells. 



Mirror Diaphragms. — In Dr. J. E. Smith's ' How to see with the 

 Microscope ' (p. 53), a reference is made to " slot diaphragms of dif- 

 ferent widths, covering the whole surface of the mirror, and only 

 allowing light to pass through the slot in such a direction that very 

 sharp shadows by oblique light will be produced." A mirror dia- 

 phragm of this kind has been sent us by the Bausch and Lomb 

 Optical Co., and is shown in fig. 114. It is made of ebonite and fits 

 as a cap over the concave mirror, the ebonite being moulded concave 



