THE SIMPLE MICROSCOPE 5 



sometimes useful, as it can be held to the eye while 

 the hands are left free to manipulate the object. 

 The Coddington lens used to be held in high esteem ; 

 it is a thick, double-convex glass grooved about the 

 middle, the groove being blackened and acting as 

 a diaphragm. It gives fair magnifying power, but 

 has some imperfections ; thus, it does not yield a 

 sufficiently large and flat field, and the working 

 distance between lens and object is too short. It 

 has to be remembered that working distance lessens 

 with increase of power, and that the shorter this 



Fig. 2. — Aplanatic Magnifier, showing Arrangement 

 OF Lenses. 



distance is, the less is the size of field and the degree 

 of illumination of the object. Lenses of the Cod- 

 dington pattern cost from 4s. 6d. to about 9s. For 

 splendid definition, size, and flatness of field, and 

 good working distance, the Aplanatic or Platyscopic 

 lenses are the best in the market. They consist of 

 three lenses cemented together, and are to be had 

 of different powers and ia various mountings to 

 suit the purchaser's requirements. Mr. John Brown- 

 ing, 146, Strand, London, supphes in pocket moimts 

 with powers ranging from 10 to 30 diameters ; 

 Messrs. W. Watson and Sons, Ltd., 313, High Hol- 

 born, W.C, supply in three patterns of mounting, 



