AERANGBMENT FOR TRAKSPARENT OBJECTS. 



169 



the field of the microscope, without killing them. It is where a 

 firm but graduated pressure is required, for the flattening out of the 

 bodies of thin semi-transparent animals, without the necessity of re- 

 moving them from the field of the microscope, that the compresso- 

 rium is most useful. Wherever the first and simplest of the above 

 methods can be had recourse to, it is the preferable one ; since 

 the object, when on a glass slide, can be subjected to the Achro- 

 matic Condenser, Polariscope, Oblique Illumination, &c., with 

 far more convenience than when removed to a plane above the 

 stage, as it must be when the aquatic box is used. Whether 

 the object be submitted to examination on a slip of glass, or in 

 the aquatic box or compressorium, it must be first brought 

 approximately into position, and supported there, just as if it 

 were in a mounted slide. The precise mode of efiecting this 

 will differ, according to the particular plan of the instrument 

 employed ; thus in some, it is only the ledge itself that slides 

 along the stage ; in others, it is a carriage of some kind, whereon 

 the object-slide rests; in others, again, it is the entire platform 

 itself that moves upon a fixed plate beneath. 



86. Having guided his object, as nearly as he can do by the 

 unassisted eye, into its proper place, the Microscopist then 

 brings his light (whether natural or artificial) to bear upon it, by 

 turning the mirror in such a direction as to reflect upon its 

 under surface the rays which are received by itself from the sky 

 or the lamp. The concave mirror is that which should always be 

 first employed, the plaTie being reserved for special purposes ; 

 and it should bring the rays to convergence in or near the plane 

 in which the object 

 lies (Pig. 54). The 

 distance at which it 

 should be ordinarily 

 set beneath the stage, 

 is that at which it 

 brings parallel rays 

 to a focus ; but this 

 distance should be 

 capable of elongation, 

 by the lengthening of 

 the stem to which the 

 mirror is attached ; 

 since the rays diverg- 

 ing from a lamp at a 

 short distance, are not 

 so soon brought to a 

 focus. The correct 

 focal adjustment of 

 the mirror may be judged of, by its formation of images of 

 window-bars, chimneys, &c., upon any semi-transparent medium 

 placed in the plane of the object. It is only, however, when 



Fig. 54. 



Arrangement of Microscope for Transparejit Objects. 



