22 METHODS OF MICROSCOPICAL RESEARCH. 



centric circles already referred to ; by bright day- 

 light adjust the eyes to some object twelve inches 

 away, and without moving the eye insert at a dis- 

 tance of four inches a card inscribed with black 

 circles, when a yellow and blue colouring will be 

 plainly discerned. 



In order that the reader may thoroughly under- 

 stand the error of refrangibility, the picture afforded 

 by the passage of a solar ray through a prism of 

 glass may be thrown upon a screen ; the rays are 

 deflected unequally, the red least and the violet 

 most. 



It may be advisable here to state that the degree 

 of dispersion of the rays of white light depends 

 upon the medium through which the ray passes, 

 and this amount of dispersion is measured by the 

 distance of the most prominent dark lines in the 

 spectrum from each other. The diamond disperses 

 much less than crown glass, while the deflection of 

 the ray is greater ; but this is a subject beyond the 

 scope of the present essay. 



Now, beside these errors, there are others to 

 which the microscopist should devote special atten- 

 tion ; they are caused by small opaque particles 

 existing in the transparent media of the eye-ball. 

 These cast their shadow on the retina, and produce 

 images which appear to exist outside the eye. 

 These extra-retinal images often appear as globules, 

 bacteroid- shaped bodies, or strings of minute pearls, 

 and may be studied by directing the eye to a sheet 

 of strongly illuminated opal glass, through a small 

 aperture made with a fine needle in a piece of thin 

 blackened cardboard. 



When the microscope is used in a vertical position, 



