22 THE MICROSCOPIST. 



optical laws, concerning refraction of light, allowing the 

 eye to see an object under a larger visual angle ; so that 

 the power of a simple microscope is in proportion to the 

 shortness of its focal length, or the distance from the lens 

 to the point where a distinct image of the object is seen. 

 This distance may be measured by directly magnifying 

 an object with the lens, if it be a small one, or by casting 

 an image of a distant window, candle, etc., upon a paper 

 or wall. The focus of the lens is the point where the 

 image is most distinct. Different persons see objects 

 naturally at different distances, but ten inches is consid- 

 ered the average distance for the minimum of distinct 

 vision. A lens, therefore, of two inches focal length, 

 magnifies five diameters ; of one inch focus, ten diameters ; 

 of one-half inch, twenty diameters ; of one-eighth inch, 

 eighty diameters ; etc. 



Simple microscopes are now seldom used, except as 

 hand magnifiers, or for the minute dissection and prepa- 

 ration of objects. They are used for the latter purpose, 

 when suitably mounted with a convenient arm, mirror, 

 etc., because of the inconvenience of larger and otherwise 

 more perfect instruments. 



Single lenses, of large size, are also used for concentra- 

 ting the light of a lamp on an object during dissection, or 

 on an opaque object on the stage of a compound micro- 

 scope. 



There are imperfections of vision attending the use of 

 all common lenses, arising from the spherical shape of the 

 surface of the lens, or from the separation of the colored 

 rays of light when passing through such a medium. 

 These imperfections are called respectively spherical and 

 chromatic aberration. To lessen or destroy these aberra- 

 tions, various plans have been proposed by opticians. For 

 reducing spherical aberration, Sir John Herschel pro- 

 posed a doublet of two plano-convex lenses, whose focal 

 lengths are as 2.3 to 1, with their convex sides together ; 



