THE AMERICAN MONTHLY 



[January, 



searching in it for living microscopic 

 animals, must have noticed that when 

 there is a superabundance of decaying 

 vegetable matter in the aquarium — 

 that is, when there is only so much 

 decay taking place in the water as will 

 not interfere with the health of the 

 higher animals inhabiting it — the mi- 

 croscopic animals are found in the 

 greatest abundance ; The Polyzoa and 

 Tubular Rotifera, especially, are found 

 in the best condition under these cir- 

 cumstances. 



"The winds of October and No- 

 vember drive a large quantity of dead 

 leaves and other lifeless vegetable 

 matter into the ponds, which, decaying, 

 form a black, offensive ooze. ... In 

 this ooze the prevailing forms of ani- 

 mal life will probably be." 



The same writer also states that 

 Chara and Nitella may also be ob- 

 tained in the depth of winter. This 

 is not in accordance with our experi- 

 ence, but we do not venture to contra- 

 dict a statement made so directly. If 

 any of our readers do find these plants 

 living in cold weather, we would be 

 pleased to learn of it. 



The Podura Scale. 



BY CHARLES STODDER. 



The excellent article on the Podura 

 scale in the December Journal de- 

 -serves the attention of microscopists, 

 and should induce them to study this 

 famous test. It is true it is not a test 

 of resolution, like finely striated dia- 

 toms, or Nobert's and Roger's rulings, 

 but it is a test, and a good one, 

 of definition. Richard Beck's figure 

 of the scales of Lepidocyrtus ciirvi- 

 colis is the earliest correct one I know 

 of. The late Prof. J. Bacon told me 

 that he had seen the same appear- 

 ance (with Spencer's objectives) long 

 before Beck published his article. 

 Dr. Pigott was the first, I believe, to 

 question the accuracy of Beck's obser- 

 vations, and his paper on the subject 

 caused quite a discussion among Lon- 

 don microscopists. I had accepted 

 Beck's figures as correct, and they are 

 :accurately correct as to appearance by 



central light. I tried Pigott's illumina- 

 tion by oblique light, but could never 

 obtain confirmation of his theory of a 

 beaded structure. There is, to be sure, 

 an appearance of beads, as there is in 

 almost every object consisting of mi- 

 nute particles of transparent matter. 



A few months ago I received a 

 letter from an esteemed correspon- 

 dent, in which he referred to the 

 " spines " of Lepidocyrtus scale. I 

 have fine specimens of it, and when I 

 sat down to my tube to refresh my 

 memory of it, I obtained the exclama- 

 tion marks perfectly according to the 

 usual representation. Then I used 

 oblique light with varied angles of in- 

 cidence ; at the best illumination the 

 so-called " spines" totally disappeared. 

 It is my belief that " spines " project- 

 ing from a surface, as these have been 

 said to project, cannot be made to 

 disappear by changing the direction 

 of the illumination. In the place of 

 the exclamation marks, there were 

 longtitudinal lines, or ridges, with 

 more or less unequal expansions, 

 producing the appearance of spines 

 or exclamation marks. In short, Dr. 

 Woodward's theory, published some 

 years ago, is the correct one of the 

 structure of the Podura scale. The 

 spines have no tangible existence. 

 Your paper has a more important 

 bearing on microscopy, animal and 

 vegetable histology, than the study of 

 the insect scale would indicate. I re- 

 fer to the fact which you mention ; 

 that different instruments give differ- 

 ent images. This I was aware of, but 

 it is the first time I have seen it men- 

 tioned, and in this case it is conclu- 

 sive, for you mention several observers 

 all using the same instruments at the 

 same time, under the same conditions. 



Is it not the same as with Podura, 

 that all complex structures will pre- 

 sent different appearances with differ- 

 ent objectives and eye-pieces ? Emi- 

 nent observers who have used one set 

 of lenses for years know only what 

 they see with them, and it is but a 

 fair inference that much of their work 

 " has got to be done again." 



