on K 



85 



ON THE MINIMUM VISIBLE. 



By A. A. C. Eliot Merlin, F.R.M.S. 



(Read October 27th, 1914.) 



I have read with great interest and profit our President's 

 Address on " Organisms and Origins." The subject is one that 

 must fascinate every microscopist, whatever his line of research 

 may be. In the address a point was raised respecting the 

 minimum visible, it being stated that " it seems impossible to 

 obtain any precise information as to the size of the smallest 

 particles that can be seen with the microscope." 



Now, setting aside the ultra-microscope, as our knowledge is 

 very exact and definite indeed on this subject, it may prove of 

 interest to deal with the question at some length. As a matter 

 of fact, when a particle properly illuminated is just visible under 

 a given objective, if the aperture be cut down by means of an iris 

 diaphragm placed above the back lens so that the particle just 

 ceases to be visible, and the numerical aperture to which the 

 objective has been thus reduced is measured, then the dimensions 

 of the particle can be exactly ascertained from the antipoint table 

 published by Mr. Nelson in the Journal of the Royal Microscopical 

 Society. This antipoint table should prove invaluable when 

 accurate and minute measurements are necessary, but little 

 interest has been apparently evinced in the matter since micro- 

 metry of a high order is no longer practised, in England at least. 

 Leaving this for the present, I venture to refer to and examine 

 the claim made by Mr. Brown at a recent meeting of the Club 

 that he had seen central " pores " on the surface of the frustules 

 of certain diatoms ; which he estimated at l/200,000th of an inch 

 in diameter. On reading Mr. Brown's " Notes on the Structure 

 of Diatoms," * I examined a specimen of Pleurosigma balticum, 



* Journ. Q.M.C., Ser. 2, Vol. II. p. 317. 



