136 SUMMARY or CURRENT RESEARCHES RELATING TO 



every point in physiological and pathological investigations. It will 

 do no harm to recall the number of times it has at this meeting stood 

 as a sentinel at the entrance to the temple whose mysteries we are 

 seeking to explore. Has not the question so tersely put by Dr. 

 Gleason at the Elmira meeting of this Society, ' Do we see what we 

 see, or don't we see what we see,^or do we see what we don't see ? ' been 

 the stopping place of more than one important issue raised at the 

 meeting ? I hope I do not need to say that I have no personal ends to 

 serve in an inquiry in which I happen to be a personal factor. Let us 

 then have a test which will for ever set at rest this vexed question of 

 resolution, I submit for your consideration the following outline of 

 a test which I venture to think will be sufficient and conclusive. Let 

 Mr. Fasoldt rule three plates under as nearly the same conditions as 

 possible, except in the number of lines in the different bands of each 

 plate. Let him label each plate and accompany it with a full descrip- 

 tion of the number of lines in each band. Let these plates be sent to 

 any gentleman in whom the great body of microscopists have con- 

 fidence as eminently qualified to conduct an investigation of this sort, 

 such as Prof. H. L. Smith of Geneva, or Col. J. J. Woodward of 

 Washington. Let whoever receives the plates remove the labels of 

 Mr. Fasoldt, and put in their place labels whose signification is 

 known only to himself. Then let the gentlemen who think they 

 have resolved 152,000 lines to the inch take the plates, make their 

 count of the lines in each band, and send in their report. Let the 

 plates also be photographed, and let the number of lines be counted ; 

 then let the results of these investigations be published. If all 

 substantially agree in the count, this will end further discussion. 



The limit of visibility of single particles of matter under the 

 Microscope bears an intimate relation to the limit of naked-eye 

 visibility. My attention was first called to the smallness of this limit 

 by an accidental circumstance. I had ruled a micrometer upon a 

 thin cover-glass consisting, as I supposed, of moderately coarse lines. 

 After several vain attempts to discover traces of the lines ruled, I 

 chanced while holding the glass at a certain angle with respect to the 

 source of light to breathe upon it. At the instant the film of moisture 

 was passing off, I was surprised to be able to see all the lines which 

 were ruled, 100 to the inch, with the greatest distinctness. I then 

 carefully filled the lines with graphite, when they were, after the 

 closest inspection, found to be as fine as any I have ever ruled. 

 According to the nearest measurement I could make, their width was 

 about 1/6 of a micron. Repeated observations gave in every case 

 satisfactory evidence of visibility. In order to ascertain what efiect 

 the thickness of the glass might have upon the visibility, the cover- 

 glass was lightly cemented to a glass slide with guttapercha, when 

 it was found that the lines were by no means as distinctly visible 

 as before. The cover was then removed, when the original obser- 

 vation was easily confirmed. The lines of this plate were readily 

 seen by Professor Pickering, and by several assistants connected 

 with the observatory. Unfortunately the glass was broken in an 

 attempt to mount it upon a brass slide. While it is a simple 



