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



mounting of the objective and making a pinhole in it where 

 we wish a sheaf of beams to pass. 



Make one such hole in the centre and examine a slide 

 containing a variety of diatoms, as for example one of the 

 show slides made with diatoms from St. Peter, Hungary. In 

 order to be able to predict the result, we must deal with a 

 specific case. Let, then, our objective be the half-inch 

 apochromatic with which R, the radius of iinnge x, is about 

 8 millimetres. Let us further suppose that the hole in the 

 card is 1 mm. across. Hence the sheaf of beams the puncta 

 of which lie within this hole, contains beams of which cl, the 

 distance between their puncta (see § 34, p. 506) may on scale 

 X be as much as one-eighth of 0*65 (the grasp of the objective) , 

 i. e. d is nearly 0*08 on scale X. Put this into the equation 



o- = \'d 



and put \ = 0'6fi, wmich is close to the wave-length of the 

 brightest rays in lamplight. We find then 



for the spacing of the finest ruling which can be produced by 

 the light passing through the small hole. This would admit 

 of detail upon the object being seen dowm to about the size of 

 a speck half the diameter of a disk of human blood. 



Accordingly all the large features upon most diatoms can 

 be seen through this small hole, as may be verified by passing 

 a slide containing a variety of diatoms under the objective. 



Now make another similar hole at some distance from the 

 centre, suppose in a position corresponding to g = 0'6 ; or, 

 still better, make two holes at that distance on opposite sides 

 of the centre. This gives us three holes in a straight line at 

 equal intervals. Then close the iris diaphragm under the 

 condenser until the sheaf of dioptric beams just fills the 

 middle hole. Only diffracted light will then appear in the 

 other two. Now pass the diatoms again under the objective, 

 and on many of them a ruling will be seen, viz. : on all those 

 which furnish diffracted light of sufficient strength in the 

 positions of the two lateral holes. This ruling will have a 

 spacing calculated by 



Here then we have actually in view one of those rulings which 

 go to build up the ordinary microscopic image — that image 



