136 NOTES ON THE CONSTKUCTION AND DESIGN 



The principal parts of the metallurgical microscope may now be 

 considered in succession. 



(a) The Stand. — There is no reason why the shape of the medical 



or biological microscope should be slavishly copied in the 

 construction of metallurgical instruments, whilst there are 

 many reasons for choosing a different form, especially when 

 there is a possibility of large specimens being examined. 

 The tripod form of foot, so convenient in work by trans- 

 mitted light, is awkwardly in the way when examining 

 metals and having occasion to use the rackwork movement 

 for raising and lowering the stage. The Jackson foot is 

 better, and a heavy horseshoe foot still better, as heavy 

 specimens, such as rail sections, may be laid on it for 

 examination under low powers. This is further facilitated 

 by making the bracket which holds the stage capable of 

 swinging to one side, and leaving a clear space between 

 the objective and the heavy horseshoe foot, as in the old 

 vertical Reichert microscope. Special forms of foot, as in 

 the Beck-Rosenhain microscope, have the advantage of 

 great rigidity in both the vertical and horizontal positions. 

 This stand is the most rigid of those examined. The design 

 of Sauveur's universal Metalloscope is also unconventional, 

 and appears to be good, but the writer has no actual experi- 

 ence of it. For photographic work the form adopted in the 

 Zeiss-Martens instrument and in Watson's horizontal micro- 

 scope is both convenient and steady. 



The inverted stand, due to Le Chatelier, has been 

 copied by several makers, but the construction is apt to be 

 flimsy, and the writer has found great difficulty in moving 

 even small specimens on the stage without altering the 

 focus, the light arms which support the optical parts being 

 liable to whip. This could perhaps be overcome by better 

 engineering design, and the type is certainly preferred in 

 some works on account of the rapidity with which specimens 

 can be inserted and examined. The optical conditions of 

 this form are discussed below. It is probable that for 

 the larger instruments to be used for photography the 

 ordinary type of stand might be departed from entirely, 

 and an arrangement modelled on the optical bench adopted, 

 the various optical parts and specimen carriers being sup- 

 ported in such a way as to move freely along a heavy bar 

 of geometrical form to preserve alignment. 



(b) Codixc Adjiixiinini. — The rack and pinion should be geo- 



metrically cut in metal of sufficient hardness to withstand 

 prolonged usage without working loose. The improvement 

 in the methods of gear cutting in engineering practice has 

 been so great in recent years that much would be gained 

 by adopting the methods of marine engineering shops in 

 the instrument maker's workshop. In large instruments, 

 the length of travel might well be greater than at present, 

 so as to allow of a wide range of objectives, and stops 

 should be provided at the ends of the rack to prevent over- 



