72 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



tions may be continuously maintained on one spot, the changing position 

 requires readjustment of the instrument. The Microscope is, therefore, 

 equipped so as to meet all these requirements. It will be noticed that 

 all the milled heads are grouped on one side of the instrument so as to 

 be readily accessible. The mechanical movements include : — coarse 

 and fine adjustments to focus, rackwork adjustment to lower and raise 

 the body, horizontal screw adjustment. In each case the mechanical 

 movements are amply sufficient for the purpose in view. A Watson- 

 Conrady condenser vertical illuminator is fitted at the side of the Micro- 

 scope and built as a part of it. For ordinary visual work the 4-volt 

 lamp is sufficient ; but for projection purposes it will be necessary to 

 use an arc-lamp, and the makers have specially adapted the " Argus " 

 arc-lamp for the purpose. It can be used in any supply circuit and re- 

 quires only four amperes of current, suitable resistance being of course 

 inserted. Fitted to the lamp is an aplanatic bullseye of such a 

 diameter that, when it is in proper position, the parallel beam of light 

 which issues from it will keep the vertical illuminator filled with light 

 during all the vertical movement that would be necessary in observing 

 the testing-piece. For projection purposes it will generally be found 

 that a sheet of cardboard, or of linen, free from creases and about 

 24 x 18 in., set up at a distance of (> or 7 feet from the Microscope 

 eye-piece, will be the most suitable. It is found that the structural 

 changes which take place in the test-piece are of great importance, the 

 information thus conveyed being not only very interesting but very 

 valuable. 



Watson and Sons' Micrometer Microscope.* — The production of a 

 Micrometer Microscope that will give readings of the accuracy that is 

 invariably desired is fraught wibh complications and difficulty. Many 

 instruments are made which are stated to give measurements to several 

 thousandths of an inch, but they frequently fail to give even accurate 

 readings in hundredths, not only on account of infinitesimal backlash 

 and strain in the screw movements, but also because of the lack of exact 

 co-ordination between the divided scales and the pitch of the screw, on 

 the drum heads of which fractions of the scales have to be read. 

 Messrs. W. Watson have produced an instrument (fig. 7) which they 

 hope will satisfy the utmost demands in respect of accuracy. It will be 

 noticed that the Microscope is made on the same principle as an ordinary 

 instrument in the upper portion. It has coarse and fine adjustments to 

 focus, the vital portion being the stage. The subject to be measured is 

 placed between adjustable jaws A, and in order to save time, a quick- 

 acting screw B is provided, by means of which a long subject can be 

 moved rapidly across the field to the extreme point that is to be set. 

 The horizontal movement is then effected by means of a standard micro- 

 meter screw C, which may have a thread in millimetres or inches, reading 

 to TT yVo m - or too mm - T ne sca l e D exactly corresponds with the 

 screw. It is verified to be in correspondence with the screw thread, 

 that is, if the screw thread is reputed to produce a movement of ? V mcu 



* W. Watson and Sons' Special Catalogue : Microscopes and Accessories for 

 Metallurgy, pp. 19-20. 



