48 ON THE ANGULAR APERTURE OF OBJECT-GLASSES. 



experience, I had thus, as nearly as 1 could, imitated, on a 

 large scale, the valve of a Gr/rosic/ma. On examination of the 

 slide thus prepared under the microscope, I found it covered 



/g\ with dark spots or dots, surrounded by a very lumi- 

 (S^^f-^ nous margin or ring. This may even be seen if the 

 ^-OS' slide is laid on white paper and closely examined ; 

 ^S' each indentation produced in the layer of balsam 



will instantaneously be followed by a shadow or dark spot on 

 the paper, with its halo. 



Now, if the markings had, in this instance, been as close 

 and near to each other as is the case in the valve of a Gyro- 

 sigma, for instance, this experiment would have lost a great 

 deal of its interest, because, in that case, the luminous rings 

 would have become confluent, if I may borrow this expression, 

 and invisible, by being lost in, and forming the general illu- 

 mination of the undotted portion of the layer ; but, as seen 

 now, each opaque spot has it own halo, which is of course 

 produced by the interception of the rays corresponding to the 

 depressions, which, instead of passing through them, emerge 

 at their margins, thus forming a luminous ring, leaving the 

 depressions themselves dark. Now, be it well observed, this 

 is the identical set of rays which, according to Dr. Griflfith's 

 theory, is refracted out of the field altogether ; and it is evi- 

 dent, if that had been the case in this instance, the luminous 

 rings would not have been formed at all. But in order fur- 

 ther to prove that no rays are tilted out of the field proceeding 

 from the depressions, 1 drew out before the blowpipe a small 

 glass tube in a very fine hair-like filament, and this delicate 

 condensor I held directly over the opaque depressions, with- 

 out, however, receiving any evidence of rays issuing there- 

 from, while the portion above the halo was likewise brilliantly 

 illuminated. This is, I think, the most direct way of 

 disproving Dr. Griffith's hypothesis. If the phenomena 

 witnessed in this instance are the same as occur in the exa- 

 mination of the valve of a gyrosigma, the manner in which 

 the markings are displayed and rendered more or less dis- 

 tinct, accordingly as the aperture of the object-glass is large 

 or small, finds an easy and natural explanation in the differ- 

 ence of the aperture itself, and its ordinary operation. The 

 luminous rings are formed mainly of oblique rays proceeding 

 from the lowest point of the depressions upwards and round 

 them ; the greater the aperture, the more oblique rays enter, 

 and the greater the contrast, and vice versa. If we depress 

 the object-glass gradually, we can trace the rays down to the 

 point from which they proceed — the dark %y>o\ gradually dis- 

 appears, and is at last replaced by a very brilliant point, from 



