48 THEORY OF THE MICROSCOPE. 



extraordinary sharpness. With sunlight the spectrum extends 

 from A to the two lines If, and appears as though covered with 

 innumerable fine lines. 



The best part of the spectral eye-piece is the ingenious apparatus 

 for measurement contrived by Browning, which is applied to the 

 side tube at h. It consists of (1) a mirror i, which throws the 

 light into the tube ; (2) a blackened plate u, on which is drawn 

 a bright cross formed of two lines at right angles ; (3) a lens z, 

 which may be so adjusted that the image which it forms of 

 the cross is reflected by the upper surface of the analyzing prism 

 to the eye of the observer ; (4) a screw /, by means of which 

 the blackened plate u can be so moved that the image of the 

 cross traverses the whole spectrum, and can therefore be adjusted 

 to any particular point of it. As this screw is connected with 

 a micrometer milled-head, it is possible to determine what adjust- 

 ment, for instance, corresponds to the Fraunhofer line B, or C, 

 &c. In order to determine conversely the position of a given 

 line in the spectrum (e.g., of an absorption-line of a solution of 

 chlorophyll) it is only necessary to set the cross to it and read 

 off' the position of the screw on the milled-head. 



The " spectral apparatus for Microscopes " of S. Merz, of Munich, 

 is of a simpler construction, but it is at the same time less 

 effective. It is made after the model of the English apparatus 

 above described, though with the omission of the arrangement 

 for measuring, and of the special stage and mirror for the com- 

 parison of the objects. The latter must therefore be adjusted in 

 front of the aperture o upon a separate stand, and the comparison 

 of their absorption lines with those of the object under observation 

 is the only method of determining the data required. Allowing 

 for this defect, which is appreciable only in particular circum- 

 stances, the eye-piece of Merz will be found to answer every 

 purpose. 



3. MEANS OF DIVIDING THE PENCILS OF KAYS. 



For some time past English and French opticians have made 

 multocular Microscopes, by which several persons are enabled to 

 observe one and the same object simultaneously. Although the 

 importance of such instruments for science is more than doubtful, 



