516 Dr. G. J. Stone j on Microscopic Vision. 



consequence slope more one way than the other. The dark 

 specks will then not be vertically over the bright ones, but 

 will have shifted a little in the direction of the more inclined 

 dotted Hues of the figure. From all which we may enunciate 

 the two following propositions: — 



Proposition 8. 

 Wlien the image of minute detail is produced by a triplet of 

 beams, or by tico pairs of beams, in each meridian, then the 

 conditions are usually such, especially ichen the detail presents 

 the appearance of round specks, that it will suddenly change 

 from bright to dark, or vice versa, upon a slight change of 

 focus ; and under special circumstances which are occasionally 

 met with more than one of these alternations may occur. 



Proposition 9. 



The conditions are likely to be such, unless special precautions 

 have been taken, that on a slight change of focus the minute 

 detail upon the object ivill appear to shift its position i\ 

 to the general position and broader features of the object. 



38. Experiments illustrating the last section. — All the con- 

 ditions spoken of in the first paragraph of the last section 

 can be reproduced if the objective be a good half-inch 

 apochromatic with Gr=0*65, and if the object be the large 

 variety of Navicula lyra which is frequently met with in slides 

 of diatoms from St. Peter, Hungary. We shall suppose the 

 diatom to lie horizontally in the field of view, i. e. with its 

 length in the direction sometimes called East and West. 

 Now insert a stop under the con- 

 denser, which will allow the sheaf of 

 beams u to pass. The figure re- 

 presents image x, which is seen by 

 taking out the eyepiece and looking 

 down the tube of the microscope. 

 It is well to use a blank eyepiece to 

 keep the eye central — that is, the 

 mounting of an eyepiece without the 

 lenses, and with a small eyehole. Image x. 



The transverse rows of specks upon 



the diatom, which lie vertically in the field of view, produce 

 the vivid spectrum v ; the longitudinal rows, which are fewer, 

 closer, and more wavy, produce on these accounts the fainter, 

 more distant, and more diffuse spectrum w. On replacing 

 the eyepiece the specks are seen on the diatom with great 

 distinctness. If the stop is in the position represented in the 



