The Use of a Simple Microscope. 27 



habit has too often led to the propagation of erroneous statements, 

 and the only safe way of learning is by a careful studi/ of objects 

 themselves as the preliminary step, any reference to drawings or 

 engravings being reserved for after comparison and verification. 

 Always bring the object to be thus studied into the centre of 

 the field of view. 



The best practice is to commence upon any common objects 

 that may be met with, such as seeds, sand, table-salt, »Scc,, 

 examining them separately at first, and then mixing them so as 

 to obtain a view of several objects together. Portions of the 

 flowers and leaves of plants afford an almost unceasing fund of 

 instruction, whether in their healthy or diseased state. 



It is always desirable to obtain first Avith a low magnifier, a 

 good general idea of the structure or composition of any object 

 before submitting it to higher powers, and to increase the 

 enlargement by degrees — using first an inch-and-a-half object- 

 glass, then a two-thirds or half-inch, a quarter-inch, &;c., as 

 required. 



To facilitate the changing of these object-glasses, and to 

 prevent the loss of time that would arise from having to screw 

 each one off and on separately each time a different power is 

 required, a contrivance called a " double nose-piece " is very 

 often had recourse to. This consists of two arms attached to 

 the lower part of the body of the instrument, having one power 

 attached to each end, and, being made to turn on its centre, 

 either one may be brought into its place by merely causing the 

 arms to describe one-half of a circle, and thus either power may 

 be changed for the other in a few seconds. Treble and quad- 

 ruple nose-pieces are also made, but are not so commonly used 

 as that which carries two powers only, the lower power serving 

 also to "find" the object for the higher one. 



It is important to have a microscope always ready for use at 

 a moment's notice. Fig. 24 is shown as fixed down upon a 

 board having a circular groove intended to admit the rim of 

 a glass shade, which, together with a piece of wash-leather thrown 

 over the eye-pieces, would be a sufficient protection from dust, 

 and only require lifting off to leave the instrument fit for use. 

 A wooden cover, or a bag made of glazed calico, might be 

 adopted as an inferior substitute. 



The diseases of plants are so intimately associated with the 

 laws of vegetable life that a superficial acquaintance with 

 the ordinary external appearance of disease in either roots, stems, 

 or leaves, cannot be of the slightest avail unless something be 

 known of the principles of vegetable physiology — i.e. of the 

 structure and function of the several parts of the plant in their 

 " normal," or natural and healthy state. To acquire this know- 



