€20 SUMMAKY OF CUEEENT RESEARCHES RELATING TO 



and that of the slower wave parallel to the optic axis, then the phase- 

 retardations in the preparation and in the qnartz wedge are placed in 

 opposition. It is possible, therefore, to pnsh the wedge into such a 

 position that these opposite retardations are equal and neutralise one 

 another. At this cross-section of the wedge a black band will appear, 

 whilst to its right and left are coloured bands similar to the colours of 

 their plates. When this adjustment is attained, the before-mentioned 

 retardation difference in the preparation is read off. In order to attain 

 extreme accuracy without too long a wedge, three interchangeable 

 wedges are supplied corresponding to retardation-differences in the 

 proportion of 0-2, 2-S, and 8-39. The first wedge consists of two 

 quartz wedges whose axes are perpendicular to each other, and a scale- 



FiG. 98. Fig. 99. 



■division corresponds in their case to a retardation difference of 0*01 /x, 

 thus enabling 0"001 /x to be easily read. The other wedges are similar 

 in principle. The line selected as standard is the green quicksilver line 

 X = 546 fi fi. 



Determination of the Properties of Objectives, — A. E. Conrady 

 gives the following report of a demonstration by F. W. Watson Baker 

 at the Quekett Club on May 17, 1907, dealing with the determina- 

 tion of the equivalent focus of objectives by Abbe's method, with the 

 measurement of their numerical aperture, the difference between ordinary 

 and compensating eye-pieces, and with methods of testing the correction 

 of objectives. 



Determination of the Equivalent Focus of Objectives (according to 

 Ahhe). — By this method two observations of a stage micrometer with 

 different tube-lengths are made to yield the true equivalent focus of an 

 objective and also the position of its upper focal plane. The principle is 

 clearly shown in fig. 100, where Q represents an objective producing an 

 image A B of an object a h. As A B is a sharp focused image of a b, it 

 follows that all rays of light passing from points in the object through 

 the object-glass are re-united in the corresponding points of the image, 

 and therefore any single ray of light proceeding from a point in the object 

 through the object-glass, is a geometrical locus of the image, and is 

 sufficient to determine its size at any given point of the optical axis. 



