510 Dr. G. J. Stoney on Microscopic Vision. 



the length of the diatom to be placed horizontally. Now the 

 nearest of the diffraction-spectra which such sources of light 

 would produce are disposed round a direct ray z, as in the 

 first of the diagrams on p. 509. Accordingly, when z is 

 in the neighbourhood of a' of the second figure, it produces 

 one, and only one, diffraction-spectrum, that at a, which comes 

 within the grasp of the objective. Similarly the direct beams 

 which have their puncta near V produce diffracted beams 

 with their puncta near b. And in the same way the beams 

 which are concentrated into those puncta of image x which lie 

 near c' and d! will produce diffracted light which reaches c 

 and d. But the direct beams that reach the intermediate 

 positions at e' } /', g f , and li' throw all their spectra too far 

 out — vertically sideways or diagonally — to be caught by the 

 objective. Hence this intermediate light cannot help to show 

 the specks, and only produces diffused light tending to veil 

 them from view. The image will therefore be cleared by 

 blocking this light out. This is easily done by cutting a 

 cross out of card, and placing it 

 over the central stop. What is then 

 seen at x is depicted in the annexed 

 figure, for in fact the diffracted 

 light is so faint, and the direct light 

 so strong, that to see each macula * 

 of diffracted light it is best to shut 

 out all but one of the maculae of 

 direct light ; and even then the 

 diffracted light at c and d is faint and Image x. 



diffuse enough to be difficult to see. 



It now only remains to adjust the draw-tube with extreme 

 care (since the objective is fully corrected only for one 

 length of draw-tube) and to apply a compensating eyepiece 

 of sufficient power. A suitable magnification is 2000, for 

 eyes that see very minute objects with ease ; and if there be 

 in the observer a slight defect in this respect, the power 

 3000 f will be found better. The power 2000 causes the 

 vertical rows — those which stand at right angles to the mid- 

 rib of the diatom — to appear with the closeness of lines ruled 

 at intervals of half a millimetre from one another when viewed 



* A inacula or spot in image x is the representative in that image of a 

 sheaf of beams emitted from the microscopic object. It may he of any 

 size, and the sheaf of beams it represents are those beams of which the 

 puncta are the points of the macula. 



t The objective made use of had an initial magnifying-power of about 

 80, and the higher power was reached by a 40 compensating eyepiece 

 which the firm of Carl Zeiss were good enough to make for the writer, 

 and which he has found to be of great service. 



