84 JOURNAL OF THE [April, 



nate it only by the oblique rays from one side of the cone, and 

 we then immediately get diffraction effects. I am bound in 

 candor to say that in most of Mr. Smith's prints I recognize 

 similar effects to those which, in my own work, I attribute to 

 oblique light. It maybe that, with improved contrivances to secure 

 exact centring of objective and condenser, we shall find advan- 

 tages in the use of the wide cone. I speak now only of my own 

 experience under existing methods. The slightest turn of the 

 mirror on its axis will change light from central to oblique; and 

 I suppose we are all in the habit of doing this, so as purposely 

 to throw light through one side or segment of the condenser for 

 the purpose of studying the effect on an object of the changing 

 direction of illumination. So unstable a source of ligJit prevents 

 our knowing very exactly when the light is strictly central, and 

 makes it hard to return to any exact condition from which we 

 have departed even a little. These considerations have kept me 

 (perhaps mistakenly) in the practice of using the narrow cone 

 of light for photography, reserving my oblique light for special 

 resolutions of striation and for the professed study of changing 

 effects. 



Similar reasons have made me distrustful of dry mounts when 

 high powers are to be used upon any but the thinnest objects. Re- 

 fraction, and attendant diffraction, are so increased with increase 

 of index, or rather increased difference of index, that it has 

 grown to be a maxim with me to have the mounting medium and 

 the object as near alike in index as is consistent with the discrim- 

 ination of structure. The pale images of transparent objects are 

 those I find most truthful, for paleness is consistent with good 

 definition and resolution, whilst the brilliant pictures are apt to 

 be glittering deceptions. I fully admit, however, that it may 

 well be that with improved glasses we may add to the extent of 

 details visible upon a surface, like that of a diatom-shell, and 

 that it is possible fhat mounting in most media would obliterate 

 the finest of these details. To a certain extent we are all familiar 

 with this. A rather coarse dry shell like P. balticiim will have 

 its details instantly obliterated if water from the immersion of 

 our objective penetrates beneath the cover glass Mr. Smith's 

 print No. 50 might pass as an excellent reproduction of this 



