Dr. G. J. Stoney on Microscopic Vision, 



517 



figure, so that u and v are at equal distances on either side of 

 the vertical diameter, then on slightly changing the focus the 

 specks will not shift sideways. But on account of the 

 diffuseness and the one-sided lateral extent of w, the adjust- 

 ment of u and w cannot be the same for different parts of w, 

 nor can it be the same for different colours, and it may be 

 found impossible to find any position of u in which there will 

 not be some shifting of the specks vertically. 



In this experiment only two of the maculae in image oa 

 (corresponding to two sheafs of beams) lie in the horizontal 

 direction, and two vertically. Accordingly in each direction 

 there is (as in fig. 1 on p. 514) only one of the two sets of the 

 dotted lines in fig. 2 upon page 515. In this case there- 

 fore the conditions for the passing of bright specks into 

 dark do not exist, and accordingly on putting the microscope 

 slowly out of focus the image grows indistinct but no black 

 specks appear. 



The case is otherwise if we use a stop 

 with a hole in it as at u in the 

 accompanying figure of image x. Here 

 there are three macula?,, corresponding 

 to three sheafs of beams in the horizontal 

 position and only two vertically. Ac- 

 cordingly on putting the microscope 

 out of focus the vertical rulings (which 

 are caused by three sheafs of beams) will 

 change from bright to dark while the 

 horizontal ones (caused by only two) will not, and the resulting 

 specks in the microscopic image will become imperfectly 

 dark. 



Diatoms which show the transition from bright specks to 

 dark in perfection are those known as Actinoptychus, of which 

 a good example is almost sure to be found on a St. Peter slide. 

 With the half-inch apochromatic and with the iris diaphragm 

 below the condenser nearly closed it gives in image x a 

 ring of strong diffracted light. Two opposite puncta in this 

 ring and a punctum in the central macula belong to three 

 beams which produce one of the very numerous rulings which 

 conspire to form the specks in the microscopic image. Each 

 such trio of beams furnishes both sets of the dotted lines in 

 fig. 2 on p. 515. And as the ring is a tolerably circular one 

 the other similar trios, whether in the same or in other 

 longitudes, furnish dotted lines in fig. 2 that are not very 

 far from being equally inclined. These are conditions that 

 will produce black specks at the height represented by p in 

 Phil. Mag. S. 5. Vol. 42. No, 251). Dec. 1896. 2 



Image x. 



