426 LI3IITS OF OPTICAL CAPACIXr OF THE MICROSCOPE. 



time the basis of the collective pencils which belong to the several 

 points of the object, and of its image on the retina, and its diameter, 

 as before shown, varies in inverse proportion of the amplification. 

 But the very conditions which must be fulfilled in order to obtain 

 sharply defined shadows of objects within the eye are exactly what 

 occur here, namely, that a strong light should enter the eye from a 

 relatively small surface. 



Whoever has, at any time, attempted to illumine the field of the 

 microscope with direct sunlight, when employing a high amplifica- 

 tion, will remember the peculiar spotty appearance of the field so 

 obtained. Some of these spots remain fixed in the field, but others 

 move with the motion of the eye. The first class of spots is due to 

 dirt particles or imperfect polish of the ocular lenses ; the second 

 arises from shades caused by intervening opacities in the tissues of 

 the eye — conjunctiva, cornea, crystalline lens, or vitreous humor.* This 

 method has even been used to discover their existence, and is, in 

 truth, a very suitable one. In proportion, however, as entoptic 

 objects become more noticeable, will a greater number of finer 

 details, of microscope objects become obscured. 



A second and inevitable disadvantage arising from the narrow diver- 

 gence angle of the emerging rays shews itself in the occurrence of 

 diffraction phenomena, whereby ihe outlines of visible objects are eff'aced, 

 and at the same time doubled or further multipled. We have to deal 

 here chiefly with diff'raction phenomena as they appear when we 

 look through a minute circular opening. A bright point of light 

 (reflection of sun on the bulb of a thermometer), viewed through a 

 pin point hole pierced in a card appears as a bright disc surrounded 

 by alternate bright and dark circles. The apparent breadth of these 

 rings, reckoned from minimum to minimum, corresponds very 



nearly to a visual angle, whose sine is equal to_ where \ expresses 



the respective wave length of the light, and a the diameter of the 

 opening. The outermost rings have exactly these dimensions, the 



* But mainly from the retinal vessels, as shown by Heinrich Muller, vide 

 Wurzburg Verhandlungen, Vol. 5, Page 411.— H. E. F. 



