•30 



THE AMERICAN MONTHLY 



[February, 



The collar also allows greater range 

 in the length of the tube, and also 

 aids in the examination of dry prepara- 

 tions not properly mounted. Various 

 other advantages have been claimed 

 for it, but they are too theoretical and 

 infinitesimal in their effects to deserve 

 consideration. 



It has been said that the correction- 

 collar enables the objective to be ad- 

 justed in accordance with the accom- 

 modation of the eyes of different 

 observers, but the greatest change in 

 the adjustment that could ever be re- 

 quired for such a purpose would be 

 much less than could be made by the 

 collar, being about o.oooi mm. 



The mechanical objections to the 

 adjustment are not worthy of further 

 consideration, for it has been shown 

 that the lenses can be set sufficiently 

 well in movable fittings. As for the 

 additional cost, Prof. Abbe has said 

 that the difficulty of adjusting the 

 lenses in a fixed position at the exact 

 distances required, within fractions of 

 a hundredth of a mm., is so great that 

 the cost is about the same as for a col- 

 lar-adjustment. 



Practically, the advantage of the 

 collar- adjustment is quite illusory, 

 when the microscope is applied to the 

 study of objects the structure of which 

 is unknown. In the observation of 

 diatoms, when the proper appearance 

 is known, the correction can be advan- 

 tageously employed to get the most 

 perfect image the objective is capable 

 of giving. But in the study of histolo- 

 gical objects, the structure of which is 

 very delicate and not exactly deter- 

 mined, it is impossible to make the 

 correction perfect by experiment, 

 judging only from the clearness and 

 sharpness of the image. Too much 

 latitude is given to false or arbitrary 

 interpretations, perhaps influenced by 

 preconceived notions of what the 

 appearance should be. With a fixed 

 mounting, and corrections properly 

 made by the maker, there can be no 

 uncertainty as to the truthfulness of 

 the image for the structure under ex- 

 amination. There is only one object 



which will give the most perfect cor- 

 rection for a certain thickness of 

 cover-glass, and that is the Abbe test- 

 plate, devised to show the co-operation- 

 of all the zones of aperture. This 

 test-plate will be fully described in a 

 future number of this Journal. 



When the article by Dr. Dippel 

 was read before the Royal Microscopi- 

 cal Society, of London, several of 

 the members objected to the views 

 set forth, among them Mr. Beck and 

 Mr. Mayall. Mr. Crisp, Mr. Ingpen, 

 and Dr. Ldmunds, on the other hand, 

 expressed opinions in favor of lenses 

 without adjustment, mainly because 

 of the acknowledged difficulty in ob- 

 taining proper correction with many 

 objects; regarding the optician as 

 most capable of determining the 

 proper correction. We are inclined 

 to regard the latter view as most ca- 

 pable of support from experience ; 

 and we need only cite as a single and 

 very striking instance of the difficulty 

 in ascertaining the proper correction, 

 even among qualified observers, the 

 experiments on the Podura scale, al- 

 luded to in our last number, and more 

 fully set forth in Dr. Carpenter's 

 book, " Revelations of the Micro- 

 scope." 



Notes on Biatomacae from 

 Tampa Bay, Florida. 



BY CHARLES STODDER. 



A few weeks ago, I received a 

 small quantity of diatoms from Mr. 

 W. S. McNeil, of Mobile, from a 

 depth of eight fathoms. The collec- 

 tion was very interesting from con- 

 taining many of the species originally 

 discovered and published by the late 

 Prof. J. W. Bailey, and also from the 

 condition in which they w^ere found. 

 Many of them appeared to have been 

 severely acted upon by the chemicals 

 used in cleaning them, or by the mud, 

 or other matter in which they were 

 found. 



This condition of partial decompo 

 sition, or disintegration, produces a 



