WITH THE MICROSCOPE. 8 1 



ment, which I have not yet seen, is made by Beckmann, of Kiel, and 

 costs seven dialers, or about a guinea. 



Mechanical finger. Professor Smith has made an instrument which 

 he terms a mechanical finger, of some value for some kinds of micro- 

 scopical work (Silliman's Journal, No. 123). By an arrangement of 

 springs and levers a small bristle can be caused to move or take up 

 any minute object while it is being examined under the object-glass. 

 An object may be selected, raised from the slide, and held while a 

 clean slide is placed in position to receive it. This instrument has 

 been made by Mr. Baker, of Holborn. Dr. Maddox has suggested 

 a slight modification, which simplifies the instrument somewhat. 

 Although the mechanical finger may be of value in special investiga- 

 tions, the general observer will not require it, and the thorough 

 student will probably acquire such dexterity in handling specimens 

 while they are in the field of the microscope that he will not feel 

 the want of any mechanical apparatus. 



149. Dissecting Tissues under the Microscope with the aid of the 

 Conipressorium. In many cases the observer may desire to dissect an 

 extremely delicate structure under the microscope, for in this way much 

 information can often be acquired with reference to the exact relation 

 existing between the structural elements of the tissue. This object 

 may be gained by means of a little instrument termed a compressorium, 

 which consists simply of a convenient arrangement by which pressure 

 can be applied to an object while under examination, pi. XXI, figs. 

 135, 136. This pressure being applied gradually, the texture becomes 

 frayed out as it were, and particular structures can often be teazed 

 out from a tissue, and demonstrated more distinctly than by any other 

 method. 



The structure of the compressorium is very simple. Many dif- 

 ferent forms have been recommended, one of the simplest consisting 

 of a thick brass plate with a hole in the centre to admit the light. 

 On one side of this is the fulcrum of a lever, the short end of 

 which acts upon a circular ring carrying the thin glass to cover the 

 preparation, while to the longer arm is attached a screw, which by 

 being turned causes the thin glass to be pressed tightly upon the 

 object placed upon a piece of plate glass placed over the hole in 

 the plate of the compressorium. A more perfect form of instrument 

 has been arranged by Mr. Highley. It is represented in pi. XXI, 

 fig. 136. 



The plate glass is usually fixed in the hole in the brass plate, but 

 it is more convenient to have a ledge attached to one side, so that 

 an ordinary plate-glass slide may rest upon it. With such an arrange- 

 ment, the tissue to be examined can be placed as may be thought 



G 



