PROCEEDINGS OP THE SOCIETY. 497 



vision, a lined object will appear to have its lines closer and closer 

 together according as it is seen more and more obliquely. In the 

 Microscope, however, we have the same number of lines to the 

 inch whether the object is seen by an axial or an oblique pencil. 



This essential diflference between naked-eye and microscopic 

 vision by oblique pencils (which Prof. Abbe had been the first to 

 point out) was most important to be kept in mind, as the opposite 

 assumption had led to some of the greatest of the mare's-nests of 

 microscopy (" All-round Vision," &c.). 



The admission of this difference, however, did not invalidate any of 

 the practical illustrations which Dr. Carpenter had given. The experi- 

 ments of Mr. Claudet and Mr. Wenham, for instance, were performed 

 with objectives of low aperture. Now the difference between the 

 two modes of oblique vision varies as the cosine of the angle of 

 obliquity, so that up to the limit of angle for objectives suitable 

 for binocular work, say 40°, the difference does not exceed 1 per 

 cent. — an amount quite inappreciable by the eye. 



Dr. Carpenter said he was not sorry to find that Prof. Abbe and 

 himself were not so much in difference as he had thought to be the 

 case. With large apertures, however, the whole conditions of 

 vision were so entirely different that they could scarcely be com- 

 pared ; while, as regarded images of lines, they were so mixed up 

 with diffraction effects that the question was necessarily in a very 

 unsettled condition. If, however. Prof. Abbe was in agreement with 

 him as to apertures under 40°, then clearly there was no question 

 between them. 



The President, in proposing a vote of thanks to Dr. Carpenter, 

 said that he was sure he was in accord with the unanimous feeling of 

 all present, in expressing the gratification which Dr. Carpenter's 

 presence that evening had afforded them. 



Mr. E. M. Nelson's observations on the Bacilli of tubercle were 

 referred to by Mr. Michael, who said that Mr. Nelson had found that 

 when examined with dark-ground illumination they take the light in 

 an unexpected and peculiar manner, appearing like grains of gold- 

 dust on black velvet. The best effect was obtained by Swift's 140° 

 condenser, with stop, illuminated by a lamp having a large-angled 

 bull's-eye accurately centered and focused, and the plane mirror. 

 Excellent images are obtained by this method with a 2/3 in. and 

 1/2 in. eye-piece or a 4/10 and a 1 in. eye-piece. Mr. Nelson thinks 

 three advantages accrue from this kind of illumination: — 1st. The 

 low power by which the organisms may be studied. 2nd. The great 

 ease with which they may be detected in tissue ; and 3rd. Saving to 

 the eyes. 



Mr. Badcock described some observations he had recently made 

 on some specimens of Surirella hifrons, which showed small processes 

 similar to those found in the ArcellinsB, and by means of which the 

 diatoms seemed to be moved to and fro (see p. 352). 



Ser. 2.— Vol. IV. 2 L 



