304 RemarJcs on High-])ower Definition. [ joumi^/j'ilTiaTo.' 



substitute the lowest therefor close in the tube, and adjust the focus 

 (which the change of eye-piece requires), the beaded appearance 

 dissolves into the usual " note of admiration " markings. Another 

 appearance may be very easily obtained in the Podura — that of a 

 series of oat-shaped cells, each end terminated by a bright spherule ; 

 and with equal reason might be claimed as the real structure. 

 Probably no one has ever examined this object so carefully and 

 systematically as the late Eichard Beck. With his own hands he 

 collected hundreds of specimens in many localities and of every 

 variety of species. Some of these he gave to me, and which I value 

 exceedingly. I never once heard him express an opinion that the 

 markings were otherwise than longitudinal ribbings. The surest 

 way of deciding the question is by examining fragmentary pieces of 

 the scale. The insects are not easily obtained at this time of the 

 year, or I would offer some illustrations. 



At the conclusion of Dr. Pigott's paper he states that " the 

 surface of metals and alloys, with a power of 1000 diameters, shows 

 under reflected light particles, apparently spherical, agglomerated 

 together, with dark lines separating the particles." The plane 

 surfaces of mercury, well-polished speculum metal, or steel, show 

 no structure, but metals with an imperfect surface are full of 

 glittering points which can be develoj)ed as spherules. A broken 

 sm-face of bright pomts is by no means a practicable test for the 

 correction of object-glasses, for the numerous images interfere and 

 cause a confusion of the indication which is requu'ed from a single 

 point only. When a particle of mercury is beaten into fine dust for 

 the purpose of obtaining a very minute point of light for testmg 

 errors, a single atom is isolated, as the comas from surrounding ones 

 would embarrass the result. The broken surface of fine cast-steel 

 consists of angular fragments or crystals ; a few of the highest can 

 be seen in focus, those beyond appear as spherules. 



At page 192 of this Journal, for April, Dr. Pigott states : — 

 " I had the good fortune to discover yesterday that the median line 

 of the Formosum is formed of four parallel rows of beads about one- 

 third the size of the general beading. Every part seems compounded 

 of cohesive spherules." I refer to this as an example of how a false 

 structure may be developed in one part of an object of this character 

 by the interference from adjoining parts. Taking the entire scale 

 of the Formosum, this four-banded appearance of beads may readily 

 be shown on the median line, and it would be hard to say that they 

 did not exist ; but this Diatom is exceedingly brittle, and liable to 

 split down the centre, or close to the median line. I have a slide 

 containing numerous fractured specimens ; in one, the midrib stands 

 out quite isolated a distance beyond the broken scale. In this 

 portion, not by any means of illumination, or any object-glass that 

 I can employ, am I able to develop a beading or molecular structure ; 



