10 



THE BRITISH NATURE BOOK 



Purchase, for this purpose, a sixpenny " finder " (Messrs. Houghton and 

 Co. and Butcher and Sons stock them), and take out the lens. This, you 

 will find, will just fit inside the inner tube which is at D ; it must be put at the 

 end nearest your eye, close to the diaphragm at E. This will almost double 

 the size of the object at which you are looking. 



A Simple Microscope. 



We next deal with the use of a portion of the telescope mounted on a stand 

 as a " table " microscope. The illustration (Fig. 6) shows a simple form of 

 stand. As arranged, it possesses several powers of magnification, and its 

 maximum will be found equal to that of an objective of about T 4 ^ of an inch. 



The stand is made of wood, and any one who is handy 

 with a fretsaw can easily make it. The dimensions of the 

 one illustrated are: height, 9 inches; the top board which 

 supports the telescope tubes being 2,\ by 3 inches. These 

 dimensions can be varied, of course. The middle stage has 

 a hole in the centre I inch in diameter, and is intended to 

 carry the object. In the base is a little mirror which can 

 be bought for a penny at a Penny Bazaar mounted on 

 wood, and pivoted so that it can reflect the light upwards 

 through the hole in the object stage. 



It will be noticed that under the stage are two sets of 

 cardboard grooves. These are not really necessary, but are 

 intended for a luxury known as a " spot lens " (described 

 below), and also for diaphragms and " tinters." 



Diaphragms of various apertures, from \ to \ inch, are 

 cut hi the centre of rectangular black cards, which slide in 

 these grooves when required. Or they may be placed on 

 FIG. 6. A home- the object stage, and the object placed above them. I need 

 scarcely add that only one is used at a time ; and that its 

 object is to concentrate the light reflected upwards by the 

 mirror upon the object under observation. Actual experiment will show in 

 which pair of grooves the card should be placed for any particular object. 



The spot lens consists of a bull's-eye lens on the flat side of which a little 

 disc of black paper is stuck centrally. As various sizes of discs may be required, 

 it is well to " stick with a lick," and not with any more adhesive liquid. 



The bull's-eye is placed on a slip of glass which fits these grooves, and the 

 proper distance from the object is found by experiment. The use of this 

 simple spot lens is to obtain what is called " dark-ground " illumination, the 

 effect of which on many objects is to make them appear brilliantly lit on all 

 sides against a background of rich dark velvet. The bull's-eye can be obtained 

 for 2d. from any shop where electric torches are sold ; or a cycle shop will 



