22 HOW TO WORK 



piece of thin plate glass (^ of an inch thick) placed at an angle of 

 45 clegs. In this way loss of light was avoided, as the magnified 

 image was seen through the glass. 



The late Mr. R. Beck about the same time adopted a similar 

 plan, using a circular piece of ordinary thin covering glass, which 

 was arranged so that the angle of inclination could be altered if 

 required. I learn from Dr. Maddox that Prof. Smith still gives the 

 preference to his own arrangement. 



These new methods of illumination which are improvements upon 

 that devised five years since by Mr. Hewitt, but on the same prin- 

 ciple, are valuable for observations upon the diatomacese. For a 

 full description the reader is referred to Professor Smith's paper in 

 Silliman's Journal for September, 1865 ; Mr. R. Beck's paper in 

 the Microscopical Journal for April, 1866 ; and the remarks made 

 by Mr. Wenham, Mr. Slack, Mr. Lobb, and others in the same 

 number. 



Mr. Dancer has proposed another modification of the above 

 plan. A little speculum, only one-sixth of an inch in diameter, is 

 introduced through a lateral aperture two inches and a half above 

 the top of the object-glass, and placed at a proper angle to reflect 

 the rays downwards (Popular Science Review, April, 1866, p. 249). 



32. Dark-ground illumination. In this place I must allude 

 cursorily to a mode of illumination which has been much in repute 

 of late years, and which is very advantageous for demonstrating some 

 structures. I refer to dark-ground illumination,) in which the object 

 appears in relief upon a black ground. In this mode of illumina- 

 tion, which is particularly applicable to investigations upon some 

 very minute organisms, such as the diatomaceae, the direct rays are 

 prevented from penetrating the specimen, and passing through the 

 object-glass, but the preparation is highly illuminated upon all sides 

 by light made to impinge upon it in a very oblique direction. Thus 

 the object is thoroughly illuminated upon every part of its surface, 

 but the ground on which it lies appears perfectly dark. There are 

 several methods by which this result may be obtained. One very 

 simple little instrument is termed a spot-glass, and consists of a 

 plano-convex lens, the convexity being so great that rays of light 

 passing through it converge with a great degree of obliquity, and 

 are brought to a focus at a short distance above the flat surface of 

 the lens, in the centre of which is placed a small circular piece of 

 black paper in order to prevent the passage of any direct rays of 

 light. The lens is fixed in a brass tube made to slide up and down, so 

 that it may be adjusted at the proper distance below the object. The 

 spot-glass may be purchased of the instrument makers for about -js. 6rf. 



