442 Messrs. Ayrton and Mather on Galvanometers. 



cone from the back lens of the objective to that point of the 

 image. Accordingly, if that object which is called the visual 

 substitute in Part L, § 14, p. 342, could be put in place of this 

 image, sending forward hemispherical waves from each point 

 of it which might be treated as uniform within the limits of 

 the aforesaid cone ; then what the Airy method as hitherto 

 applied has investigated is what minuteness of detail it would 

 be possible to see in this object. But such an inquiry does 

 not even touch the main points towards which an investigation 

 of microscopic vision needs to be directed. It tells us nothing 

 as to what this visual substitute is, how far it can represent 

 something on the microscopic object, and how far it consists of 

 intercostal markings, diffraction- fringes arising from the 

 mismanagement of the illuminating apparatus, or any other 

 misleading effects. And of course it gives us no clue as to 

 how we are either to interpret or control any of these effects. 

 Every legitimate mode of resolution, and there are number- 

 less such, must lead to precisely the same result, if we can 

 succeed in correctly following out its consequences. Where 

 they differ is in our power of handling them. Abbe's 

 resolution into beams of plane waves recommends itself above 

 all others in regard to this ; since it substitutes uniformity 

 for that w T ant of uniformity which exists in all other methods 

 of resolution in just those places where in the present state 

 of our knowledge we are unable to assign the law of this 

 want of uniformity. And even if we ever come to know 

 this law, the resolution into uniform plane waves will still 

 recommend itself in consequence of the law of uniformity 

 being simpler and therefore more easily handled than any law 

 of non-uniformity. 



[To be continued.] 



XLIII. Galvanometers. By Prof. W. E. Ayrton, F.R.S., 

 and T. "Mather*. 



IN an article on " The Electric Discharge in a Magnetic 

 Field " contributed by Sir David Salomons to the Phil. 

 Mag. for September, it is stated on p. 255 : — 



" I made a large number of experiments with galvanometers 

 built on the D'Arsonval-Deprez type, and obtained very 

 varying results by modifying the magnetic field. By in- 

 creasing the field a maximum sensibility was reached, which 

 decreased on further increasing the field. 



u The various experiments described no doubt indicate the 

 cause of this, viz. that the field being made too powerful, less 

 current passes through the coil, and the sensibility begins to 

 * Communicated bv the Authors. 



