SOME SPECIAL PBOCESSES 259 



specimen requires to be varied to bring out some particular 

 structure. 



To overcome the want of stability in the horizontal position 

 Messrs. Zeiss have recently constructed a stand -which is known 

 as Martens' stand (Fig. 71). This instrument may be used 

 only when illumination by reflected light is required, and it 

 differs in design from any other at present in use. The fine- 

 adjustment motion M controls movement of the stage and not of 

 the body-tube at all, so that in using the vertical illuminator 

 there is no alteration whatever in the incidence of the light. 

 Some modifications can be introduced to enable it to be used 

 for transparent objects ; but owing to the fact that it is only 

 available in the horizontal position it is not so convenient for 

 purposes other than photo-micrography or direct projection. 

 It must be admitted that as a stand for photo-micrographic 

 work, pure and simple even excluding its applicability 

 for metal -work it is almost ideal, as the position of maximum 

 ability is attained when horizontal ; and, owing to the fact 

 that all the adjustable parts are supported in long slides, it 

 is very difficult for any flexure to take place even during 

 a prolonged exposure. For very high-power work of the 

 most critical nature, it is exceptionally stiff and rigid. 



The general arrangements for metal-work are the same 

 as in the previously described stand. The method of control- 

 ling the fine-adjustment is a direct one. It is connected by a 

 Hooke's joint to a long arm, so that no intermediate cords or 

 pulleys or any gear-wheel arrangement for connecting up the 

 fine-adjustment is necessary. Kotation of the stage and 

 mechanical motion are both provided for, and verniers are 

 also supplied, so that any precise position of the stage may be 

 arranged for. 



Another metallurgical microscope of special design is that 

 made to the specification of Mr. Walter Eosenhain by Messrs. 

 K. & J. Beck (Fig. 72). This instrument is one of great interest, 

 as in the working out of the model no regard has been paid 

 to any previous designs of stands. The point to which great 

 attention has been paid is stability; in this respect the 

 instrument has certainly no superior, and, for one that allows 

 of use in the vertical or horizontal position, has probably 

 no equal. The limb has been designed on engineering principles, 



s 2 



