36 the president's address. 



structures be viewed through the haze at the edges of the 

 obstacle. For example, if the finger be held in front of the 

 eye at a distance of about four inches, when the vision is 

 normal, and if the eye be focussed on some object at a greater 

 distance, a haze will be seen surrounding the out-of -focus finger. 

 Now if a fine-lined object, nearly at the limit for resolution, be 

 examined, the resolution will be found to be strengthened when 

 it is viewed through the haze at the edge of the finger. A 

 black-edged card might with advantage replace the finger. 

 Some ridges, which counted 45 to the inch, on the black cover 

 of a cloth-bound book were examined, and it was found that 

 they became much more distinct when they were viewed 

 through the haze at the edge of a card. 



After this digression, let us see what Mr. Wright says about 

 the Microscopic Image. In Art. 21 (6) he attacks the Eich- 

 horn intercostals from a point of view different to that I have 

 taken, and I heartily concur in all his conclusions. The next 

 paragraph (c), with regard to the statement that the striae of 

 A. Pellucida, which Mr. Sollit measured as 120,000-130,000 

 per inch, were ghosts, is not so clear for the following 

 reasons : — 



First. — At that time there was no objective with a resolving 

 limit approaching such figures. 



Secondly. — A false ghost must always be within the resolv- 

 ing limit of the lens. 



Thirdly. — A false ghost must always be an integral multiple 

 of the true structure. 



Therefore, assuming that Mr. Sollit had a coarse A. Pellucida 

 of say 90,000 striae per inch, the coarsest false ghost he could 

 have made must have had 180,000 lines per inch, and the next 

 one 270,000, and so on. Now, as 180,000 lines per inch was 

 beyond the limit of any lens then constructed, no one had ever 

 seen a ghost of the true striae on A. Pellucida. In some dry 

 mounts of this diatom, especially those burnt on cover, there 

 are apparently coarse wrinklings of some outer membrane, 

 which have nothing whatever to do with the striae in question ; 

 these can easily be seen with any quarter-inch objective. It is 

 more than probable that some running about 40,000 per inch 

 were doubled and afterwards erroneously measured, for it is 



