50 W. LeConte Stevens — Microscope Magnification. 



but insignificant and barety discernable departures from de- 

 flections required by the law. This follows from the small 

 angles of deflections and low velocity of the motion of the 

 galvanometer magnet in the experiments. It is also to be 

 noticed that with a good magnet of the size stated, and with 

 the galvanometer making one vibration in about 5 seconds, the 

 coil with 5 turns passes over only 2 cms. or less, of end of 

 magnet in order that it shall give a deflection of 15 divisions 

 of scale. It is evident that in these conditions a very short 

 time is occupied in cutting the lines of force. If the max- 

 imum deflection used is 15 divisions of the scale, the actual 

 angular deflection of the magnets and mirror amounts to only 

 6° 29'. Yet 15 divisions are quite a length on the scale, being 

 equal to 37"5 cms. But these experiments may be as readily 

 made with a ballistic galvanometer. Then the magnets and 

 coils have to be of larger dimensions. 



Experiments similar to those given have served to graduate 

 galvanometers. We have here the means of sending definite 

 amounts of currents through an ordinary galvanometer and we 

 may thus graduate its angular readings into their relative 

 values in current. The damping of the galvanometer has, 

 however, to be applied to the readings, and then the results 

 may best be put in the form of a curve. 



Stevens Institute of Technolo°-v, Hoboken, N. J. 



Aet. IV. — Microscope Magnification; by W. LeConte 



Stevens. 



When a lens is interposed as magnifier between the eye and 

 an object, it produces a virtual image of this, the accommoda- 

 tion of the eye being so adjusted as to relax the ciliary muscle 

 and thus secure the most comfortable vision. For normal eyes 

 this occurs when the entering rays are parallel, rather than 

 when the accommodation is for the conventional near-point of 

 distinct vision. The position of the virtual image is hence 

 indeterminate ; but by common consent it has been generally 

 agreed to consider its distance on the axial line to be 10 

 inches, or 254 millimeters, from the optical center of the lens. 



It can be easily shown that, if the lens and object be fixed, 

 the increase of visual angle produced is a maximum when the 

 eye is closest to the lens. It is never possible to measure 

 accurately the distance from the optical center of the lens to 

 that of the refracting combination composing the observer's 

 eye. In theoretical calculations an allowance should be made 

 for it ; practically it is regarded as zero. 



