518 Dr. G J. Stoney on Microscopic Vision. 



fig. 2 on p. 515. Now open the iris diaphragm, insert the 

 eyepiece, and look at this diatom : it will be found to exhibit 

 the phenomenon with great distinctness. 



A modification of this experiment is to cut a piece of card 

 of the annexed form and to place it over 



the back of the objective. It allows only ^ ^^ 



two of each trio of beams to pass. These / ^_^^ \ 

 proceed to form the rulings which are I ( ) \ 

 competent to form specks in the micro- *— * *— ' 

 scopic image ; and accordingly that image 

 will still exhibit the bright specks, which, however, will now 

 go out of focus without any dark specks appearing. 



Notice that the specks now appeal" to traverse in a direction 

 perpendicular to the diameter of the semicircular stop, while 

 the microscope is being put out of focus. This is because the 

 dotted lines of fig. 1, p. 514, are here necessarily oblique. 



39. Experiments exhibiting Illusory Colouration. See Pro- 

 position 4, p. .345. — The half-inch apochromatic, of which 

 the grasp is 0*65, answers admirably for these experiments. 

 It should be furnished with a Davis's shutter, i. e. a small iris 

 diaphragm interposed between the objective and the micrc- 

 scope tube, by which the aperture may be reduced when 

 desired. We shall also want a small central stop about 8 mm, 

 in diameter, which can be put over the back of the ob- 

 jective, and which may be cut out of card. This will 

 enable us, at one stage of our experiment, to exclude the sheaf 

 of dioptric (*". e. undiffracted) beams, while leaving a free 

 passage to others. 



The object which perhaps most strikingly exhibits the 

 phenomenon we are now occupied with is the diatom known 

 as Actinocyclus JRalfsii. 1°. Select a valve of this diatom 

 which looks blue when the Davis's shutter is partly closed. 

 2°. On then opening the Davis's shutter the colour is for the 

 most part but not altogether discharged, and at the same time 

 a quantity of detail comes into view which was not visible or 

 was seen imperfectly when the colour was present. It consists 

 of specks variously distributed OAer the valve. 8°. Now 

 introduce the central stop over the back lens of the objective, 

 and open the Davis's shutter. The image is thereby con- 

 verted from blue to red, and there are added to the image 

 dark grooves and bright lunes distributed over the intervals 

 between the legitimate specks. 4°. Finally examine tie 

 diatom with the immersion objective, which has a much 

 greater grasp than the half-inch. It has now become 

 absolutely colourless, and new detail has come into view 



