22 



NATURAL HISTORY 



is perhaps on the wliole most convenient, tlie common 

 Argand lamp. Concentrate this light on the object with 

 a proper condensing lens, taking care at the same time 

 to reduce the quantity, if necessary, by means of dia- 

 phragms or stops* placed under the stage : these should 

 be rendered capable of adjustment as to distance from 

 the object, &c. so as to transmit only a cone of rays of 

 the proper dimensions. 



A magnifying power of about 100 to 500 will be found 

 to be sufficient for most purposes ; although in an inspec- 

 tion of the Monads, and some minute portions of other 

 objects, a stronger one will doubtless be required. I do 

 not think, however, that any advantage will be gained 

 from powers exceeding 800, as it is of far more impor- 

 tance to obtain a deep penetration and perfect definition 

 than an excess of amplification. Apply in the outset, 

 therefore, a low power, say 100, and if on trial it prove 

 insufficient, double it, and proceed onwards until you are 

 satisfied as to the result ; taking it as a general rule, 

 never to increase the power beyond what is ahsoluteli/ re- 

 quisite. As the expression, magnifying power, has refe- 

 rence to some standard or other for sight, it is necessary 

 that I should inform the reader of the one I have 

 adopted here ; it is the decimal one, presuming that an 

 object is always viewed without a magnifier under the 



* Sec " Microscopic Illustrations," and " Cabinet," for further instruc- 

 tions. 



