206 A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 



deducible from the six spectra of the P. angulatum. According 

 to Dr. Eichliorn's diagram,* we should see with onr widest- 

 angled lenses a hexagonal structure with intercostals, the 

 spectra being ranged opposite to the sides of the hexagons, 

 and that this was considered to be the nearest possible 

 approach to the true structure attainable with our present optical 

 means is shown by Dr. Zeiss' narrow pencil photograph of 

 this diatom x 4900, reproduced in " Carpenter," in which inter- 

 costals are visible. It is now known that with a real hexagonal 

 arrangement of the kind indicated in Dr. Eich horn's diagram, 

 easily observable with a 1-in. in the Triceo'atium favus, the six 

 spectra of the first order are not aro'anged op2)Osite the sides of the 

 hexagons, as shown by Eichhorn, but ojyposite to their angles. If 

 such a mistake could arise and remain so long uncorrected and 

 unexplained in Professor Abbe's deliberately-selected specimen 

 of a computation of a comparatively simple regular formation, 

 the probability of error when dealing with more elaborate 

 irregular structures need not be dwelt upon. 



Having touched on the four causes which, I believe, may be 

 fairly taken as principally militating against the general adop- 

 tion of critical methods by ordinary workers, and, it is hoped, 

 having shown how easy it is for each individual microscopist 

 to practically test their eflicacy and trustworthiness for himself, 

 in conclusion I venture to call your attention to a few points 

 connected with the " glass and brass " department, which seem 

 to have been hitherto overlooked by those whom they would 

 more especially benefit. 



The first of these relates to a new form of Huyghenian 

 eyepiece computed by Mr. Nelson. I have had one of these 

 oculars, made by Messrs. Powell and Lealand, magnifying 12 

 times, in constant use for the past two years, and have tested 

 it on a large variety of low and high power, dry and oil- 

 immersion apochromatic, semi-apochromatic, and old achromatic 

 objectives. Be the explanation what it may, I have found that 

 with all these so differently corrected objectives, with one exception 

 only, it has proved itself decidedly superior to both the compen- 

 sating and Huyghenian forms, the image appearing more crisply 

 defined, and diatomic " dots " and structures exhibiting markedly 



* Carpenter's "The Microscope," 7th and 8th editions, p. 71. 



