Dr. G. J. Stoney on Microscopic Vision, 341 



convergence upon it, after the reversal, of two or more of 

 the undulations of uniform plane waves into which the light 

 emitted by the object may be resolved. 



10. Of course other resolutions than the two hitherto 

 considered — that into spherical waves thrown off from the 

 several points of the surface of an object, and that into plane 

 waves thrown off from the surface as a whole — are possible : 

 and in fact, if a resolution of the disturbance in the sether 

 between the object and the objective of a microscope is made 

 into plane waves, these will become curved while passing 

 through and after emerging from the objective ; and it is as 

 curved waves that they reach and produce the microscopic 

 image. They, in fact, become convex waves that are nearly 

 spherical. The centres of these nearly spherical waves are 

 obviously the points of the focal plane (or rather, focal sur- 

 face, for it is slightly curved) of parallel light incident on the 

 objective. This focal plane lies between the objective and the 

 microscopic image, and in all the cases that need to be con- 

 sidered it lies near the objective, and therefore sufficiently far 

 from the microscopic image to render the curvature of the 

 waves where they reach that image but slight. 



11. Magnification. — Let us now return to the standard 

 image. It is of the same size as the object. If we could by 

 any contrivance increase the wave-lengths of the light that 

 forms it — if, for instance, we could make the wave-lengths a 

 thousand times larger, making them the same fractions of a 

 millimetre which actual light-waves are of a micron — wo 

 should in this way enlarge the image 1000 times, since the 

 interference of the longer waves coming in the same directions 

 as before would produce rulings all of which would be 1000 

 times coarser than before. This enlarged image would 

 obviously contain precisely the same amount of detail as the 

 standard image. 



This method of enlarging an image is only practicable on a 

 small scale, since we can but slightly increase wave-lengths 

 (as when we place the object in a highly refracting medium 

 and its image in the air) ; but what is very much the same 

 result may be brought about in another way, viz., by dimi- 

 nishing the inclination of the beams of plane waves to one 

 another, without altering the lengths of the waves ; since the 

 ruling which results from the interference of two such beams 

 may be made coarser either by lengthening the waves of 

 which each beam consists, or by diminishing the inclination 

 of the beams to one another. 



12. Useful work done by the objective. — The useful part of 

 what is accomplished by the objective of a microscope is that 



