On Ghost Images. By A. A. C. E. Merlin. 647 



ascertain their true size, the antipoint correction for a working 

 aperture of 0'9 must be added ; this amounts to 0' 00000324 inch, 

 making the actual diameter of the holes ^^3-^011 iiich. 



We tlius find that the best modern optical combinations are 

 capable of accurately picturing the minute image formed by a 

 diatomic aperture §3500 inch in diameter, the tiny disk visually 

 appearing singularly well defined, although small even under a 

 a power of 3200. 



The C. asteronvphcdus is, to the critical microscopist, one of the 

 most interesting and useful forms of all the Diatomacepe, As a 

 test object of the working quality of objectives of the highest 

 numerical aperture yet constructed, it can hardly be surpassed, 

 for any lens which will show the structure of the perforated caps 

 brilliantly, with good contrast, and no trace of fog under a nearly 

 full axial illuminating cone, must be well corrected and adjusted, 

 and can be relied upon to afford trustworthy results when employed 

 upon such objects as the smaller infusoria, bacteria, etc. It also 

 serves to demonstrate the difference which a slight change in tube- 

 length or movement of the correctional collar of a good objective 

 will effect towards the making or marring of the image, and will 

 speedily convince the tyro that the old-fashioned rough-and-ready 

 plan of setting the collar about half way between " covered " and 

 " uncovered " on a fixed tube-length could only lead to the pro- 

 duction of faulty pictures devoid of detail. Owing to this cause 

 there can be little doubt that much of the work done and recorded 

 by biologists in the past on objects such as the smaller infusoria, 

 monads, bacteria, and other minute organisms requiiing large 

 aperture skilfully utilised to reveal their proper forms and char- 

 acteristics, will require revision by future observers. The 

 demonstration of these minute creatures taxes the powers of the 

 Ijest modern optical appliances to the utmost, yet how many of 

 the recognised " authorities " on their functions, appearance, and 

 structure, would be regarded as experts in any question involving 

 critical microscopical manipulation ? The typical bacteriologist 

 may be quoted as an example. He will choose and belaud a lens 

 and optical aiTangement which will show the greatest number of 

 bacilli which happen to be lying haphazard througiiout a large 

 field of view all equally distinctly in focus at the same moment. 

 The advantage of a lens which will only effect this is not very 

 evident, for, as a rule, no particularly important object is attained 

 in being able to see a number of separate forms distributed over 

 a large area all exactly on one focal plane, and this is fortunate, 

 ibr it will be found in practice that lenses of the very highest 

 perfection, when their full aperture is utilised, do not yield nearly 

 such flat fields as many objectives of inferior resolving power. The 

 scientific investigator, in studying the features of a minute 

 organism, may be supposed to require the lens and manipulative 

 methods which will reveal its form and ultimate structure most 



