266 THE UlSToKY AND DEVKU >P.MEXT ( >E THE MICKOSCOPE 



that it may be placed on the stage of the microscope in a perfectly 

 horizontal position. With a view of compensating for small de- 

 ficiencies in the parallelism of the two surfaces, the stage is provided 

 \\ itli tin- screws, SS. by \vhich means it may be tilted, and the upper 

 surface of tin- object made to lie in a truly horizontal plane, which 

 of course i> m-crary in order to place the entire field in the focus 

 of the instrument. The stage is a mechanical one, the milled heads, 

 T'" and T"", imparting to it a forward and backward movement and 

 a lateral movement respectively. 



After the source of light has been placed in the most desirable 

 position for the examination of a certain specimen, if a sample of 

 different thickness be placed on the stage, the microscope must be 

 lowered or raised, with the result that the light is no longer in the 

 proper position and must again be adjusted. To avoid this trouble- 

 some manipulation, the stage of the microscope is made adjustable 

 in height by turning the milled head T". When the object is too 

 thick to be placed on the stage, the latter may be turned to one side 

 and the preparation laid on the foot of the microscope. For still 

 larger pieces of metal, the stage may be removed altogether, the 

 body of the instrument turned around 180, and the metal placed on 

 the table by the side of the stand; or the body of the microscope is 

 connected directly with its foot, for which purpose the intermediate 

 piece bearing the stage must be removed. 



Prof. Rejto's method for the preparation of the sample is a> 

 follows : 



Tl le piece of metal to be examined has two of its sides planed oft" 

 and made parallel. The upper surface is polished until it is free 

 from scratches. It is then washed with absolute alcohol, and wiped 

 with a, soft clean cloth in order to remove all fatty substances. The 

 polished surface is next surrounded with a. layer of wax so as to form 

 a rim projecting a little above the surface. Being placed horizon- 

 tally, pure concentrated hydrochloric acid is poured over it to a 

 depth of about three millimetres, and allowed to act for five minutes. 

 It is then poured off, and the surface covered with concentrated 

 ammonia. The \\a.\ is removed, and the surface wiped dry with a 

 soft cloth. A little oil is next poured over it and allowed to remain 



for lil'teeli Illillllte-. 



It is then dried again and rubbed on a piece of chamois leather 

 until it assumes a shiny appearance. 



When large piece> of metal are to be examined, small portions 

 must In- polished by hand and etched as described above. 



Ki--. -JUS ,-nid -JlHI are phot omicrogra pi i> taken with this instru- 

 ment, which are self-explaiiat or\ of the nature of the work it does. 



Tank microscopes (a IM> called ai|iiarium microscopes) have, for 



certain kind- of work, a value of their own. They may be used 



with lou powers outside the glasv ,.r above the water; or the 



object glass may be protected by a water tiuht tube outside it, and 



with a discol'glas> lixed (also water tight) into that end of the tube 



vliich stands below the front lens of the objective, at a proper 



for the focus, may then be plunged into the aquarium. 



Indeed, the t U be of t lie instrument may be so protected as to work 



