1881.] 



MICKOSCOPICAL JOUKNAL. 



209 



same sort of proof was given with 

 regard to this organism as in the for- 

 mer case. And, lastly, a form of ery- 

 sipelas was shown in the ear of rab- 

 bits caused by the inoculation of the 

 rod-shaped bacillus of the septicemia 

 of mice ; this, sometimes, though not 

 always, killed the animals. 



The importance of these experi- 

 ments can scarcely be estimated at 

 present, but there is no doubt that 

 they show a great advance, and no 

 work has more tended to throw light 

 upon the complicated subject of 

 pathogenic bacteria than that of Dr. 

 Koch. Dr. Koch lays great stress on 

 the value of micro-photography as es- 

 sential to an accurate record of facts ; 

 and photographs, which he exhibited 

 on Friday, were certainly very fine 

 examples of what can be done in this 

 way. — Medical Tiines and Gazette. 



The Ablbe Binocular Eye-piece. 



[The following abstract from a 

 letter written by the author to his 

 brother, Mr. C. F. Cox, is given, by 

 permission. The eye-piece is descri- 

 bed on page 221 of Volume I. — Ed.] 



My Zeiss binocular eye-piece came 

 about a fortnight ago,^ and I have 

 spent several evenings over it very 

 satisfactorily. In fine, it is like the 

 sectional drawings which were given 

 in the Journal of the Royal Micro- 

 scopical Society and in Hitchcock's 

 Journal. It is provided with loose 

 caps which go on over the eye-pieces. 

 One pair of these is pierced with 

 semi-circular openings, and the other 

 with circular ones. 



When the caps are turned so that 

 the openings are in the outer half of 

 the eye-pieces, the vision is stereo- 

 scopic. When the openings are in 

 the inner half the vision is pseudo- 

 scopic. When the circular openings 

 are used the vision is binocular but 

 not stereoscopic. This is the form in 

 which it is useful for high powers. 

 The field is only of the size usual 

 when continental stands are used, and 

 is considerably smaller than that even 



of Beck's " Popular." The semi-cir- 

 cular openings make a little awkward- 

 ness of vision, but one quickly be- 

 comes used to it. By reason of the small- 

 ness of field this binocular is not so 

 pleasant to use with the low powers 

 as the Wenham. In every other re- 

 spect it works as well. 



With high powers its work is so 

 pleasant and admirable that it is a 

 great delight to use it. I find the 

 best results with central light and 

 Webster condenser, the latter used 

 with a longitudinal slit for admitting 

 the light. In this way I actually 

 found that I could go quite as far in 

 resolution on the test-plate as with 

 the single tube and central light. The 

 Powell & Leland yi resolved all the 

 first 17 tests of Moller with this strict- 

 ly central illumination. This is more 

 than I have ever done with the same 

 glass and central light. I account 

 for it by supposing that the use of 

 both eyes in this way, with the longi- 

 tudinal-slit-stop underthe condenser, 

 really gives a degree of oblique light 

 by the crossing of the rays. There is 

 a beautiful finish in the definition 

 which makes the result upon ordinary 

 diatoms more satisfactory than any 

 other I have ever had. The partial 

 stereoscopic effect seems to give the 

 markings a degree of relief which 

 makes them sharper and inore real 

 than ever before. It seems to me a 

 demonstration of the fact that the 

 dots are areolae and not spherules. I 

 don't think the beauty of its work 

 with high powers and central light 

 can be understood without seeing it. 



With oblique light and high powers 

 I went through the test-plate ; but 

 using the Wenham disc-illuminator, 

 I found I could not get the two im- 

 ages to coincide in the case of long 

 shells like Nitzschias. Whether I 

 can overcome this difficulty I don't 

 know. I will try the prism next, to 

 see if there is the same trouble. 



On the whole it is a great advance, 

 and goes far to establish my old pro- 

 position that a binocular eye-piece is 

 the true binocular for the microscope. 



