Dr. Gr. J. Stoney on Microscopic Vision. 525 



beam, and g is the grasp of the associated diffracted beam. 

 [The regularity of the detail on this diatom throws nearly 

 all the diffracted light into definite spectra.] We have made 

 the convention that g and g' shall be positive when on oppo- 

 site sides of the centre. Accordingly, when on the same side 

 we must regard g' as negative. Now open the iris diaphragm 

 until the macula of the dioptric sheaf of beams has expanded 

 so as just to touch those of the diffracted sheafs of beams. 

 Then what is seen on looking at image x is either what lies 

 within the greater or what lies within the less of the two 

 large circles of the figure, of which the outer one corresponds 

 to 1*3, the grasp of the objective, and the inner one to a 

 grasp = 1. If the diatom is " in optical contact,''' that is if the 

 layer of air between it and the cover- glass is less than the 

 thickness of Stokes's layer, then what is seen extends to the 

 outer circle. Whereas if the chink between the diatom and 

 the cover-glass is more than the thickness of the Stokes's 

 layer, no light can get into the cover-glass except such as 

 passes up through it and the oil at le>s than the " critical 

 angle," and this supplies light in image x, only within the 

 smaller circle which corresponds to Gr (or NA) = 1. The 

 reason of all this is obvious from what is stated in § 19 ; and 

 very interesting appearances may be obtained by traversing 

 the slide sideways and thus bringing diatom after diatom 

 under the objective. With some the light will extend in its 

 full intensity to the outer circle. These are they thai; are in 

 good optical contact. With others nothing is seen beyond 

 the smaller circle. These are they that lie beyond the thin 

 Stokes's layer which lies like a varnish on the under side of 

 the cover-glass. And now and then one may be found in 

 which the outer ring of light is present but dim. This is 

 one the interval between which and the glass is nearly the 

 full thickness of the Stokes's layer. The whole experiment, 

 if made with green monochromatic light, is one of exceeding 

 beauty. 



42. How to See the Rulings.-^- It has been explained that the 

 microscopic image is produced by the interlacing and mutual 

 interference of luminous rulings, each of which extends over 

 the whole image field ; and that each ruling is due to the 

 concurrence of two or more beams the puncta of which are 

 situated at equal intervals along some straight line upon 

 image x. In order, however, to get light enough to see a 

 ruling, we must be content to use small sheafs of beams 

 instead of individual beams, and these in image x become 

 small maculae instead of points. This in practice may be 

 accomplished by putting a disk of card over the back of the 



