34 



On the Measurement of the Working Focal Length of Object- 

 Glasses, and their Magnifying Power. 



By G. West Roybton-Pigott, M.A., M.D., Cantab , M.R.C.P., 

 M.R.I., Fellow of the Cambridge Philosopbical, tlie Royal 

 Astronomical and Microscopical Societies, late Fellow of St. 

 Peter's College, Cambridge. 



Though strange, it is nevertheless true, that two observers, with 

 the same eyepiece and objective, do not always see an object magni- 

 fied to the same amplitude. A change of focus may be necessary. 

 A short-sighted person sees the virtual image of the magnified 

 object at a distance of perhaps six inches ; the long-sighted adjusts 

 it perhaps at twelve or even eighteen inches ; the distinct plane of 

 vision, called the field of view, is variably placed according to the 

 focal length of the eye of the observer, and therefore at distances 

 of considerable variety. Under these circumstances, if two ob- 

 servers, the one very short, and the other very long-sighted, both 

 agree to observe together, their powers of vision proportionably 

 vary. It is necessary, then, to fix a standard for estimation. 

 Most persons can see distinctly at ten inches distance. In this case 

 the power of any lens at this distance is found by dividing ten by 

 the focal length / or 



Magnifying power =^10 -f- focal length/ 



or=^orlO -T-/. 



To persons unread in optical principles, there appears some little 

 difficulty in understanding the variable foci of a lens, and I may, 

 perhaps, be excused for introducing the following illustration, known 

 I believe to very few working opticians : — If we take a lens (say 

 3 inches focus), and form the image of a candle (or much better, the 

 image of a small perforation in a brass plate placed before it) upon 

 a sheet of white paper, it is well known that as you move the candle 

 from the lens you must move up the paper towards it, in order to 

 obtain a clear image. Now, the special point which I wish to 

 bring before the notice of the members of the Quekett Club is 



