106 MTCKOSCOPES FOE 



productive of very aeciu'ate results. It is as follows : — Having set 

 up the microscope and screwed in or adapted the glasses which are 

 intended to be used, take a glass micrometer, and place it on the 

 stage in the same manner as if it were an object to be viewed, then 

 carefully adjust the focus of your instrument, so that the lines on the 

 micrometer may appear quite sharp and distinct. Next, take a 

 common ruler, or a slip of cai'd-board with equal divisions of some 

 known measurement drawn upon it, every tenth division being longer 

 than the rest, and fix it 20 inches from the eye, whilst looking through 

 the microscope ; then, whilst one eye is directed to the rule or cai-d- 

 board, and the other to the lines of the micrometer, seen in the 

 microscope, ascertain how many on the card are equal to a given 

 number on the micrometer. If the divisions on the latter be 1 -100th 

 of an inch, and one of them be equal to ten on the card, it is clear 

 that every division on the card will represent 1-lOOOth of an inch. 

 Thus, when the micrometer shall be removed, and an animalcule be 

 put into its place, if the creature subtend five divisions on the card, 

 its size in linear measure will be 5-lOOOth of an inch. Note — The 

 glasses must not be changed diuing the cxpeiiment, nor their dis- 

 tances apart ; neither must the distance between the card and the 

 eye be in any way altered. 



Sectiox IV. — On Glass Tiibes, ^c, for iahing Infusoria from the 

 Water, and placing them in the Apparatus for examination. — As these 

 useful little contrivances, (which have been before alluded to in page 

 96) are drawn and described by me in the Microscopic Caiinet in 

 1832, it wiU be necessary merely to mention that little or no im- 

 provement has been made uj)on them since that period, excepting 

 perhaps that a, finer description is found to answer the purpose better 

 than when the larger ones are drawn out at their extremities in the 

 manner there proposed. The reader will find figui-es and descrip- 

 tions of several useful little contrivances in a work entitled 

 '■'■Microscopic Objects — animal, vegetalle, and mineral.'" 



Section" V. — On the Compressor, or Crush Box. — The last i-emark 

 is equally applicable to the Aquatic-live-boxes, which were described 

 in the Illustrations, 1828, and subsequently their different modifica- 

 tions. In order to form an idea of a compressor, or crush-box, you 

 must sujiposc that the cover of the live-box is so adapted to its box 



