26 ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 



cannot omit here to express his cordial thanks to Professor Abbe 

 for the kindness with which that gentleman explained and 

 personally demonstrated to him these most important and 

 interesting experiments ; as, also, for more recent communications 

 by letter, which enable him to offer the present account to the 

 readers of the Bristol Naturahsts' Society's Proceedings. 



Arrangement of the Instrument : — 



The simplest form of monocular instrument unencumbered with 

 substage, or any appliance beneath the stage, except the usual 

 diaphragm, is to be preferred : perfect freedom of movement of the 

 mirror in all directions being all that is required for convenient 

 handling. 



Objective : — 



For these experiments the power used should be such as will 

 give a distinct image of the ruled groups of lines employed as 

 *' object,'' without straining the eye. An inch or three quarters of 

 an inch, with an object containing lines ruled so fine and close as 

 to be well seen when magnified 50 to 60 times will be convenient. 

 The writer uses Zeiss's aa system (ij-inch focal length, 20** 

 aperture) and magnifying with a low eyepiece 40 ; but an amplifica- 

 tion of ^o to 60 obtained with a stronger eyepiece appears to suit 

 best (with good illumination) the groups of lines prepared expressly 

 for use with the aa system of Zeiss. An inch lens of any good 

 maker answers equally well. 



Arrangement for shitting-off portions of the working area of the 

 lens : — 



Whatever objective be used, its setting must admit of the easy 

 introduction and removal at will, of diaphragms occupying the 

 plane of upper focus of the lens, i.e., just behind the back lens of 

 the system. For the low power used, a correction screw collar 

 is not wanted 3 and the screw thread, by which the objective is 

 connected with the nozzle of the instrument, should be close to 



