7 6 



HA RD WICKE'S SCIENCE- G OS SI P. 



simple pencils, as the distance for the lamp corre- 

 sponding to the focus f 2 , if we can calculate the 

 distance L /and deduct it from ld, we shall obtain 

 the required distance / D. The distance / G must be 

 less than fg, or the refracted rays will not be 

 divergent. Let us suppose that a pencil of con- 

 venient size for the mirror is formed when the 

 point / is \ inch nearer to G than F is. F G and / G 

 will then both be known. If / G be called u, and 

 L G be v, v - u will represent the required distance 

 l /. Let F g be called p. 



may greatly affect the character of the ultimate 

 pencil. 



Divergent pencils enable us to set the mirror at 

 extreme distances from the object, and so to obtain 

 an ultimate pencil whose rays are very little oblique 

 to the axis of the microscope. The compound form 

 has in certain cases the advantage of giving a more 

 brilliant illumination, by admitting of a nearer 

 position for the lamp. 



(To be continued.) 







*~ L 



Fig. 33- 



Then V = 



P U 



Therefore V — U = . 



V- 



which 



p-ir " ' " ~p-u' 



gives us this rule : Square the exact distance 

 betweeen the flame and bull's-eye. Divide the 

 product by the difference between that distance and 

 the focal length of the bull's-eye. Deduct the 

 quotient from anyone of the lamp distances for simple 

 pencils, and the remainder will be the distance for 

 the lamp suitable to that form of compound pencil. 



Example. — Suppose the bull's-eye to be a small 

 one, having a focal length on its convex side of 

 2 inches, and that it stands at a distance of if inches 



. ('■•-75)' = 3'o6g5 



•as -25 



= 12 '25 inches. If from any of the longer distances 

 in the table, \2\ inches be deducted, the remainder 

 will be the distances for the lamp when the bull's- 

 eye is in the position supposed. Were the bull's-eye 

 \ in. further from the flame, the necessary deduction 



from the flame. V— U will then be 



would be 



' i ' 87O 2 



- = 28J inches, a difference of result 



125 



so startling, that it may be specially commended to 

 the notice of any tyro who supposes that success can 

 be obtained without attention to minute details. An 

 ■error of ]-inch in the position of a small bull's-eye 



THE CHICKWEED WINTER GREEN 

 (TRIENTALIS EUROP&A). 



By R. S. Wishart, M.A. 



SOUTH of the Tweed this plant is very rarely 

 found, and when it does cross the Border it 

 appears to be confined to the hills about Cumberland 

 and York, where it exists but sparingly. In Scotland 

 it is not abundant till you cross the Forth, at least, 

 and get into Perthshire, and if you wish to find a 

 plentiful supply you should stray in some of the parts 

 of this or its neighbouring counties eastward, and visit 

 certain localities where botanists and ramblers from 

 large towns go less frequently. Any little that we 

 have seen concerning this pretty plant in Britain 

 appears to have been written from an English point 

 of view, and leads one to suppose that the species is 

 much rarer than it really is, and that the habit of the 

 plant is more constant than as a matter of fact it is 

 found to be. Both of these conclusions are likely to 

 be arrived at from examining a few casual specimens, 

 but if you want to see the plant in all its glory and to 

 know its habit aright you must set yourself in some 

 such place as where our specimens were gathered, 

 where all the hilly woods for miles around are literally 



