THE ANIMAL KINGDOM. 9 



standing such an extraordinary result, everything was seen 

 with great clearness. 



Even the measurement of the most minute microscopic 

 details has acquired a degree of precision surpassing every- 

 thing that could be imagined. There are glass micrometers 

 in which each millimetre * is divided into 500 parts, or 

 lines, of such tenuity that the most practised eye cannot 

 make them out. This is effected by means of an instru- 

 ment of extreme delicacy, which only works in the dead of 

 night, when there is nothing to disturb it or impede the 

 accuracy of its tracings. For t-his purpose, the workman 

 himself does not enter his work-room ; a piece of mechanism, 

 moved by clock-work, at a suitable hour sets the machine 

 in movement. The invisible divisions of the glass plate are 

 engraved by means of an excessively fine diamond point, 

 which is found to be totally worn out when its work is ac- 

 complished. 



But the means of investigation at the disposal of the 

 microscopist do not end here. In observations of extreme 

 delicacy micrometers are called into requisition which are 

 constructed with an ingenious mechanism capable of divid- 

 ing a millimetre (say Ath of an inch) into 10,000 parts by 

 moving spiders' threads with the aid of a simple screw. 

 The investigator also utilizes in a thousand ways both sim- 

 ple and polarized light, as well as chemical re-agents; and 

 as these last, owing to the vapors they disengage, injure 

 the glasses and dull their surface, those who work with the 

 microscope, in order to avoid the inconvenience thus occa- 

 sioned, employ particular instruments, the lenses of which 

 are placed below the objects to be magnified. 



1 Equal to .039371 inch. 



