266 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



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 

 with the screws, S S, by which means it may be tilted, and the upper 

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

 of course is necessary 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 ; pr 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 as 

 follows : 



The 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 wax is removed, and the surface wiped dry with a 

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

 for fifteen minutes. 



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

 until it assumes a shiny appearance. 



When large pieces of metal are to be examined, small portions 

 must be polished by hand and etched as described above. 



Figs. 208 and 209 are photomicrographs taken with this instru- 

 ment, which are self-explanatory of the nature of the work it does. 



Tank microscopes (also called aquarium microscopes) have, for 

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

 with low powers outside the glass or above the water ; or the 

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

 with a disc of glass fixed (also water-tight) into that end of the tube 

 which stands below the front lens of the objective, at a proper- 

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

 Indeed, the tube of the instrument may be so protected as to work 



