Dr. Gf. J. Stoney on Microscopic Vision. 511 



by the unassisted eye from a distance of ten inches. When 

 all the foregoing precautions are taken the observer is 

 rewarded by seeing something like 17 or 18 specks in each row 

 where the rows are longest, and fewer in other places. The 

 specks are fainter than the rows, because from their being some- 

 what closer, the diffracted light at c and d lies farther out than 

 that at a and b, and less of it is grasped by the objective. It 

 is also^more diffuse, and therefore less easily seen in image x, 

 partly because the specks are disposed in wavy lines longitu- 

 dinally while they are arranged in nearly straight lines across 

 the diatom. Another reason why it is fainter is that the longi- 

 tudinal rows are much less numerous than the transverse. 

 On all these accounts it is very difficult to see the maculae 

 at c and d in image x. 



Another point is well illustrated by this experiment. The 

 larger features on the diatom require a certain moderate 

 aperture to show them well. Now beams so divergent 

 as those that have their puncta in maculae so separated as 

 a', b ; , c f , and d' cannot concur to show these larger 

 features unless in co-operation with intermediate beams that 

 have been blocked out. The larger features are accordingly 

 only seen by the light of these maculae, each acting separately ; 

 and the sheaf of beams that reaches one of them has too small 

 an aperture to show those features well. They present 

 accordingly that clumsy blurred-out look familiar to all 

 microscopists who have worked with annular or oblique 

 illumination. 



Again, the specks themselves look very much like little 

 molehills viewed from above. This is because they are due 

 to the intersection of rulings which are of the first order, 

 and which do not differ much from one another in their 

 spacing. Rulings of the first order mean such as are pro- 

 duced by interference between only one pair of beams. The 

 law of intensity in rulings of this simplest kind is that repre- 

 sented in the accompanying figure, and therefore the specks 



which are produced by the intersection of rulings of the first 

 order must have the same kind of rounded appearance. 



Two other appearances are likely to be seen in making 

 this experiment. One is that the bright specks will become 

 dark specks on a slight change of focus; the other is that they 

 may perhaps seem to shift their positions relatively to the 

 larger features of the object when the focal adjustment is dis- 



