150 Transactions of the Society. 



rigidity results from this arrangement, and it is particularly con- 

 venient that manipulations at the eye-piece end of the tube may 

 be carried out freely without fear of disturbing the focus of the 

 objective or even — as sometimes happens — accidentally pushing 

 the body-tube bodily down upon the specimen ; where it is desired 

 to attach a small camera to the Microscope body direct, this rigidity 

 would also be a great advantage. 



In the present instrument the body is made of unusually thick 

 tube, and thus serves to support the necessary illuminating 

 apparatus by means of slides placed outside three apertures open- 

 ing at the front and both sides of the lower end of the tube. 



The Illuminator. — In all metallurgical Microscopes hitherto 

 constructed, with the exception of a peculiar instrument designed 

 by H. Le Chatelier, the illuminator has been employed as a detach- 

 able fitting ; in the present instrument, the illuminator is an 

 integral part of the body-tube. The advantage secured by this 

 means lies in the absence of a movable attachment intervening 

 between the objective and the tube and inevitably introducing a 

 certain amount of looseness. In the present instrument the 

 objective is screwed direct into the lower end of the body-tube, or 

 into a centring nose-piece directly placed in the end of the tube. 

 In addition to the gain in rigidity, this arrangement makes it easy 

 to interchange one illuminating appliance for another without in 

 any way disturbing the focus of the objective, while with the older 

 arrangement a change could only be made by removing the 

 illuminator and objective ; the new instrument thus provides a 

 valuable facility for investigating the effects of different methods 

 of lighting upon the microscopic appearance of various structures — 

 a process likely to lead to valuable results in interpretation. 



The illuminator proper consists of a short slide fitting into 

 any of the three sets of dovetails corresponding to the three open- 

 ings of the tube ; to the slide itself is attached a short swinging 

 arm carrying a spindle capable of rotation with slight friction. 

 "When the slide is in position, the spindle projects horizontally into 

 the tube, the outer end being furnished with a substantial milled 

 head. The inner end of this spindle is provided with an axial 

 hole, into which the small holders with various reflectors fit inter- 

 changeably. By moving the slide up and down, by moving the 

 swinging arm backwards or forwards, and by rotating the milled 

 head, the reflector may be placed in any position or at any desired 

 angle. These adjustments are of the greatest value in securing 

 uniform illumination and the suppression of undesirable internal 

 reflections, while special modes of illumination, corresponding 

 somewhat to the dark-ground effects with transmitted light, may 

 be obtained. For the most brilliant and uniform illumination, 

 with a minimum of internal reflections, the author finds a reflector 

 of thin silvered glass, placed so as to cover a little less than half 



