1882.J 



MICROSCOPICAL JOURNAL. 



130 



The function of these organs seems to 

 be to facilitate the males in adhering to 

 the female during sexual intercourse. 



Mr. Binder, called the attention of the 

 members particularly to objects of this 

 kind not that he claimed a new discovery, 

 but because they are so easily obtained 

 and requiring very little skill in their pre- 

 paration. Microscopical specimens of 

 rare beauty, interest, and instruction can 

 be obtained. The suckers are not the 

 only objects of interest to be found upon 

 the beetles. By dissection, the spiracles 

 and the branchial trachea of the res- 

 piratory apparatus can be shown, and 

 when these objects are made sufficiently 

 transparent, it is possible to show the 

 tracheal tubes within the boring case of 

 the legs surrounded by their delicate 

 spiral covering. 



At a meeting of the New York. Mi- 

 croscopical Society, held May 19th, 

 the subject of " Illumination of Micro- 

 scopic Objects " was discussed. The 

 President, Mr. Braman, opened the sub- 

 ject by stating the importance in micro- 

 scopical investigation of careful attention 

 to the intensity and direction of the 

 illumination. Mr. Hitchcock, was then 

 called upon to speak on the subject. 

 Referring to some remarks of the Presi- 

 dent, he stated that the best objectives 

 for the study of bacteria, were not neces- 

 sarily lenses of excessive aperture. The 

 flagella of Bacterium were discovered by 

 a Powell & Lealand, wide-angled ^, but, 

 afterward seen by the same observer with 

 a ^^ of moderate angular aperture. He 

 stated that the difficulty of defining such 

 organisms as bacteria was mainly in the 

 proper adjustment of the light ; not so 

 much due to imperfections in the objec- 

 tive, as to the manipulation. The secret 

 in defining bacteria was to have light of 

 .sufficient intensity condensed upon the 

 exact point where it is most effective. He 

 stated that there was a very simple and 

 interesting problem, which has puzzled 

 many microscopists of late, to which he 

 would call attention by the aid of a dia- 

 gram on the black-board, and endeavor 

 to explain without the aid of mathematics. 

 It is, perhaps, not easy to understand how 

 the immersion-fluid affects the resolving 

 power of a lens, while it does not change 

 the angular magnitude of the cone of 

 light. It is clear that with a front-lens y% 

 of an inch in diameter, with a given work- 

 ing-distance, the angle included between 

 the focal point and the extremities of the 

 diameter of the lens, will be the same, 



whether in air or water, or any other 

 medium. The angular magnitude is al- 

 ways the same, but by the interposition of 

 a medium denser, than air in front of the 

 objective, some of the rays which in air 

 passed outside of the angle, and which 

 therefore could not entef- the objective, 

 are refracted so as to reach the front lens. 

 Hence a greater number of rays enter the 

 lens, although the angular magnitude 

 remains the same. The illumination is, 

 therefore, better, and the resolving power 

 is increased, since the wave-lengths are 

 shortened by the denser medium. 



Numerical-aperture indicates the rela- 

 tion between focal length and aperture. 

 It also indicates the resolving power of 

 an objective in a very simple way. Tak- 

 ing the wave-length of green light as 

 suoTT the resolving power in lines to the 

 inch is given by the product of the 

 numerical aperture multiplied by 1,000,000 

 (n. a. X 1,000,000). 



Mr. William Wales, being called upon, 

 said that it was the aim of the optician to 

 constantly improve his objectives, and it 

 remains for microscopists to determine 

 what they will do. While microscopists 

 are constantly demanding better lenses, it 

 should be known that great skill is re- 

 quired to obtain the best results with the 

 lenses that are produced. He made a 

 lens long ago that would resolve the A. 

 pelluctda, but it was two years before the 

 resolution was effected, because proper 

 manipulation was wanted. 



Mr. Dinwiddle, showed a specimen of 

 " Peley's hair," a volcanic product from 

 the Hawaiian Islands, which he stated to 

 be full of elongated cavities, and crystals. 



Mr. Kunz, made some remarks about 

 a similar product from blast-furnace slag. 



Mr. Balen, showed some pond-life, 

 Plumatella, newt tad-poles, etc. 



NOTICES OF BOOKS. 



Sea Mosses a Collector's Guide and an 

 Introduction to the Study of Marine 

 Algce. By A. B. Hervey, A. M. Bos- 

 ton : S. E. Cassino, 1881. (Pp. 281. 

 and 20 colored plates). 

 The author of this book is well known 

 to the microscopists of the country as a 

 graceful speaker and writer, as well as an 

 enthusiastic student of the algae. In the 

 Introduction he declares his purpose in 

 writing it, to be to afford a " convenient 

 and competent guide" for beginners in 



