16 



THE AMERICAN MONTHLY 



[January, 



recuperate his inental and physical 

 vigor. 



We liave undertaken to publisli 

 the " Catalogue of the Diatomacese" 

 as a subscription book, to be issued 

 in four parts, at one dollar and a 

 quarter each, and an advertisement 

 of the work will be found elsewhere 

 in this Journal. 



We call attention to the publi- 

 cation in this place, because we be- 

 lieve it is of sufficient scientific 

 value to justify an extended editor- 

 ial notice; and also because we 

 wish to say that it is our intention 

 to make it, in every respect, a work 

 which shall be worthy of a place in 

 the finest library. 



As the book will not be com- 

 menced until a sufficient number of 

 subscriptions is secured to insure the 

 completion of the work, those who 

 intend to secure copies will do well 

 to subscribe immediately. 



Each part must be paid for as it 

 is issued, but no payments in ad- 

 vance are desired. 



CORRESPONDENCE. 



On Central Light. 



To THE Editor :— The following lit- 

 tle " wrinkle " has, no doubt, occurred to 

 many of your readers, but as I have never 

 seen it mentioned in any article on the 

 microscope, I thought it might be new 

 and useful to some. 



As I continually have occasion to care- 

 fully test the performance of various ob- 

 jectives, I find it of importance, in secur- 

 ing the best definition, to have the light 

 as nearly central, or coincident with the 

 optic axis of the microscope, as possible. 

 To effect this, having placed the object 

 on the stage, and focussed, I remove the 

 eye-piece, and placing the eye centrically 

 at the end of the tube, observe the image 

 of the diaphragm and mirror, projected at 

 the back of the objective. This should 

 appear as a disc of light in the centre of 

 the latter. If the spot of light is not 

 circular in outline, or does not appear 

 central with the lenses of the objective. 



adjust the mirror or diaphragm, or both, 

 until it is so. The aperture of the dia- 

 phragm may be perfectly central, and yet 

 the light which strikes the object be re- 

 ceived through only a part of the open- 

 ing, from one side of the mirror, thus 

 giving oblique illumination, and more or 

 less imperfect definition, although often a 

 better resolution of close lines. 



This adjustment of the light is so 



easily and so quickly effected and, with 



good lenses, gives such uniformly good 



results, that I find it of great advantage. 



Edward Pennock. 



Philadelphia, Dec. 6th, iSyg. 



[The full benefits of strictly central 

 light are seldom reaped by microscopists, 

 and this note of Mr. Pennock has certain 

 practical bearings which many of our read- 

 ers would do well to observe. Our corres- 

 pondent has found, by experience in test- 

 ing objectives, that strictly central light is 

 better for definition, than the light which 

 comes from the mirror carelessly adjusted. 

 Such evidence commends itself to the 

 reader's attention. In a future number 

 we shall discuss the subject of illumina- 

 tion quite thoroughly, and explain why 

 certain results are obtained by particular 

 methods. What we wish to urge now, is 

 the value of a good diaphragm. By far 

 the greater part of microscopical observa- 

 tion is carried on by what is called central 

 light, but what in truth, is not central. 

 In the course of our own observations 

 we almost never work without the small-^ 

 est diaphragm-hole in use, and it is a 

 source of wonder to us that so many 

 really excellent observers seem to be ab- 

 solutely ignorant of the advantages of a 

 diaphragm. Hardly one man in a dozen 

 appreciates its value, and yet, w:e might 

 almost say that we could tell at a glance, 

 whether a familiar object is shown with 

 or without a diaphragm. A suitably- 

 arranged diaphragm will be of more value 

 to the student, than a dozen of the more 

 elaborate traps sold to aid microscopists 

 in their work. — Ed.] 



A Note From Mr. Tolles. 



[We have received the following note 

 from Mr. Tolles, which we publish at his 

 request. — Ed.] 



Boston, Dec. 23, 1879. 

 R. Hitchcock, Esq. ; 



Dear Sir — I have your letter of 22d 

 instant. 



All I wish at your hands is the state- 



