186 PROCEEDINGS OF THE ACADEMY OF [March, 



refracted rays or complete diffracted beams enter into their image 

 formation. 



Three important questions have now been settled : Firstly, that the 

 diffracted beams from certain structures are free from refracted rays; 

 secondly, that sharp distinct images may result from such diffracted 

 beams exclusively; thirdly, that where such beams are sufficiently 

 separated from the dioptric beam to permit of their being eclipsed by 

 a suitable screen, no image of the elements producing them will be 

 visible. 



Returning to the conditions of the previous experiment, we will 

 substitute for the diatom a well-stained blood slide, preferably patho- 

 logical, containing large eosinophilous leucocytes. As this is mounted in 

 balsam, the corpuscles would be scarcely visible were they not stained, 

 hence the image must be produced principally by absorption and 

 little refraction can be expected. There must be some diffraction at 

 the margins, but the diffracted beams thus produced will be slightly 

 deflected, and for all practical purposes indistinguishable in their 

 effect from refracted rays. After examining the image through right 

 tube, and noting that practically nothing can be seen save by con- 

 trast of color, even the granules of the leucocytes being " drowned out" 

 by the wide cone of illumination, the eye should be rested for several 

 minutes in a dark place, to recover its sensitiveness not merely to faint 

 images, but, what is particularly important in this case, to color sen- 

 sation, which will have been impaired by the brilliant red color of the 

 stained blood. When this has been done, the image through left tube 

 may be inspected, and as anticipated it will be found exceedingly 

 faint. Mere sketchy outlines of the corpuscles will be seen, of a red- 

 dish tint, but what will most impress the observer will "be that the 

 granular texture of the leucocytes will be quite distinctly imaged and 

 of a bluish-gray tint. It is hardly necessary to state that this indicates 

 diffraction origin of the image if the diffracted beams are deflected 

 sufficiently to be partly cut off at margin of objective, as the distancing 

 of the granules would indicate should be the case. It has already 

 been shown why the visually red Eosin stain presents no serious diffi- 

 culties in the way of a contrasting diffraction image. 



In these experiments the position of the semicircular diaphragm 

 might be reversed so as to give the diffracted beam the advantage of 

 the direct tube, but in a good Wenham binocular it makes little differ- 

 ence which tube is used. I may be particularly fortunate in this 

 respect, but among my own binoculars this is the case, and in at least 

 two of them the difference in definition between the two images is not 



