USE OF AN ERKCTOR. 7 



as broad as it is; but a statement that the same 

 hair is magnified three hundred and sixty thousand 

 timCvS, only excites a- gasping sensation of wonder, 

 until it is ascertained by calculation that the big 

 figures only mean what the little figures instantly 

 express. In these pages the scientific plan will 

 always be followed. 



If expense is not an object, it is well to be 

 provided with an object-glass as low as two or 

 three inches focus, which will allow the whole of 

 objects, having the diameter of half an inch or 

 more, to be seen at once. Such a low power is 

 exceedingly well-adapted for the examination of 

 living insects, or of the exquisite preparations of 

 entire insects, which can now be had of all op- 

 ticians. Microscopes which have a draw tube can 

 be furnished with an erector^ an instrument so called 

 because it erects the images, which the microscope 

 has turned upside down, through the crossing of 

 the rays. This is very convenient for making dis- 

 sections under the instrument; but it also gives us 

 the means of reducing the magnifying power of an 

 object-glass, and thus obtaining a larger field. The 

 erector is affixed to the end of the draw tube, 

 and by pulling it out, or thrusting it in, the rays 

 from the object-glass are intercepted at different 

 distances, and various degrees of power obtained. 

 As it is only the object of this preliminary chapter 



