Dr. Gr. J. Stoney on Microscopic Vision* 523 



i* illustrated on our diatom by the whole inside of each cell 

 becoming full of a nearly uniform sheet of light instead of a 

 group of definite markings, when the iris diaphragm is 

 suitably opened. It is easy to see from this that there is no 

 one opening of the iris diaphragm which will, in all cases, 

 give the best effect — the effect most free from intercostal 

 markings. In each case it depends on the way the spectra 

 are disposed, and will therefore differ from one object to 

 another. Its success, so far as intercostal markings are 

 concerned, depends on the circumstance that when a small 

 dioptric macula is shifted about in image a, the illusory effects 

 undergo rapid change, while the image of true detail is but 

 little affected. Hence the real features of an object are well 

 seen with a considerable illuminating cone ; and they may 

 even be better seen on account of the admission of oblique 

 rays, as these will both add to the visible detail and will 

 diminish that defect in images which consists in the rounding 

 off of sharp edges. On the contrary, the false effects produced 

 by the several small sheafs of beams in the cone are so diverse 

 that when jumbled together they become undistinguishable. 

 This is the next best thing to their being got rid of. 



It is evident that anything which intensifies the strength 

 of the more inclined beams will give rise to brighter inter- 

 costal markings. They are, therefore, of exaggerated strength 

 when the object has been mounted in a medium of extra high 

 refractive index, owing to the effect which is described in 

 § 18, p. 346. < 



(b) The diatom employed in the foregoing experiments 

 has the detail upon it disposed with the regularity of a honey- 

 comb. It therefore concentrates most of the light diffracted 

 by it into definite spectra, and this has led to the formation 

 of intercostal markings of equal definiteness and regularity. 

 Where, however, less symmetry prevails in the disposition of 

 the detail upon an object, the light it diffracts is not distri- 

 buted according to any simple law, and a corresponding want 

 of regularity ensues among the spurious markings, which 

 may become shreds, lunes, and patches, black, white, or 

 tinted, usually twisted about, and sometimes flickering (from 

 slight movements of the observer's eye), and which are 

 chiefly conspicuous where there is some flat space in the 

 microscopic image unoccupied by real detail. Numbers of 

 diatoms exhibit these phenomena. 



(c) Another frequent event is the presence of a haze of 

 light over everything. It will occur when, from want of a 

 central stop, there is dioptric light whose corresponding- 

 diffracted light is abundant and lies beyond the grasp of the 



