Criticism of Theories of Microscopic Vision. 161 



denser be good or bad. or in or out of adjustment — in fact, 

 whatever the incident light may be, the whole of it can be 

 resolved into, and may legitimately be replaced by, the undu- 

 lations of plane wavelets of my theory. 



There seems to run through Mr. Wright's paper an 

 impression that periodic structure in an object has some 

 peculiar relation to my method of resolution. It has none. 

 The resolution of Light into uniform plane wavelets is just as 

 available for investigating what the microscope can do upon 

 the isolated objects met with in Dr. Dallinger's work referred 

 to by Mr. Wright on p. 4l>2, and in determining how the 

 appearances presented by these objects are to be interpreted, 

 as it is in dealing with the approximately periodic structures 

 which exist on portions of most diatoms. Mr. Wright cannot 

 have appreciated the fact that the way in which the plane 

 wavelets of my theory form the image of the dots on a diatom, 

 is not by any process which averages them all, but by locating 

 each separately in whatever is its proper position. 



If Mr. Wright will take the trouble of going over his paper, 

 correcting the mistakes on these and similar fundamental 

 points by which he has been misled, he will probably modify 

 profoundly the views he has entertained with reference to the 

 resolution of light into plane wavelets, and he will find that 

 there is not one iota of divergence between what the micro- 

 scopist meets with in actual practice and that which is indi- 

 cated by the true theory when correctly handled. 



Before closing this letter I wish to record my concurrence 

 with Mr. Wright in the views he has expressed about the 

 importance of pressing forward the improvement of objectives 

 in other directions than as regards their aperture, and espe- 

 cially with respect to their freedom from spherical aberration ; 

 and I may be allowed to add my admiration of the extra- 

 ordinary advance towards perfection which one meets with in 

 some picked objectives. Mr. Wright speaks of an eyepiece 

 magnifying 21 times as the highest he uses. He would find 

 the next of the series of compensating eyepieces, one magni- 

 fying 40 times, of great use in practical microscopic work 

 with such specially good objectives. 



I may also observe that there is nothing so formidable 

 as Mr. Wright apprehends in using the apparatus which I 

 employ for furnishing monochromatic light, and that it is 

 most instructive to have recourse to it on suitable occasions. 

 It is of especial service, and at the same time most easy of 

 application, when direct sunshine is available. No lenses are 

 then required. Reflect the sun's light by a heliostat on to 



Phil. May. S. 5. Vol. 46. No. 278. July 1898. M 



