520 Dr. G. J. Stoney on Microscopic Vision, 



is then formed by the ring of red diffracted light along with 

 the fainter and apparently white diffused diffracted light. 

 Accordingly the image in this case is preponderatingly red. 



At the same time the red diffracted light is now obliged to 

 act without the cooperation of the part of the light of the 

 dioptric beams with which it before produced rulings. What 

 was before a triplet of beams producing a ruling has now 

 become the two extreme beams left to operate without the 

 middle one, and they produce a ruling that is twice as fine 

 It is thus that black bands and bright patches are produced 

 between the legitimate specks. These are very conspicuous 

 in the image, notwithstanding which they are quite foreign 

 to the object. They are produced very much in the same 

 way as intercostal markings, to the illustration of which the 

 next section will be devoted. 



Similar experiments may be made with innumerable other 

 objects. Thus the familiar diatom, Pleurosigma angulatum* 

 becomes buff'-coloured when viewed with the Davis's shutter 

 sufficiently closed to exclude all but dioptric beams and that 

 inner portion of diffracted light which has its origin exclu- 

 sively in the larger features of the object ; whereas it becomes 

 of a delicate blue when seen with the dark-field illumination 

 obtained by putting a stop of the proper size under the con- 

 denser. In this case the object is seen by its diffracted light 

 only, much of which is coloured. Similarly the tubercle 

 bacillus when stained with fuchsin is intense red when examined 

 through the half-inch in the ordinary way, but becomes a bluish 

 white when seen with black-ground illumination. The study 

 of a variety of cases like the above will be found instructive. 



40. Intercostal Markings and allied phenomena. See § 15, 

 p. 345. — Light which does not contribute to delineate any- 

 thing upon the object is apt to intrude in three forms — 

 either (a) concentrated into patterns which are superposed 

 upon the microscopic image, or (b) scattered in patches over 

 parts of it, or (r) spread in the form of a luminous haze 

 over everything. We shall endeavour to exemplify each of 

 these. 



(a) The first is well sho^n in the image of Peristephania 

 eutycha when examined through the half-inch apochromatic 

 objective. 1 he real detail upon this diatom seems to consist 

 of divisions into hexagonal cells which make it look like a 

 honeycomb. Adjust the draw-tube accurately. Now nearly 

 close the lower iris diaphragm, and look at image x. The 

 dioptric light makes a central white macula, and round it are 

 displayed, as in the figure, a beautiful an ay of coloured 

 macula?, each of which is a spectrum, blue inwards and red 



