THE BINOCULAR MICROSCOPE. 



113 



In describing his improvements, he observes : " That in 

 obtaining binocularity with the compound achromatic mi- 

 croscope, in its complete acting state, there are far greater 

 practical difficulties to contend against, and which it is 

 highly important to overcome, in order to correct some of 

 the false appearances arising from what is considered the 

 very perfection of the instrument. 



"All the object-glasses, from the one-inch upwards, are 

 possessed of considerable angular aperture ; consequently, 

 images of the object are obtained from a different point of 

 view, with the two opposite extremes of the margin of the 

 cone of rays; and the resulting effect is, that there are a 

 number of dissimilar perspectives of the object all blended 

 together upon the single retina at once. For this reason, 

 if the object has any considerable bulk, we shall have a 

 more accurate notion of its form by reducing the aperture 

 of the object-glass. 



"Select any object lying in an inclined position, and 

 place it in the centre of the field of view of the micro- 

 scope; then, with a card held close to the object-glass, 

 stop off alternately the right or left hand portion of the 

 front lens : it will be seen that during each alternate 

 change certain parts of the object will alter in their rela- 

 tive position. 



" To illustrate this, fig. 65 a, b 

 are enlarged drawings of a portion 

 of the egg of the common bed-bug 

 {Cimex lecticularis), the operculum 

 which covers the orifice having 

 been forced off at the time the 

 young was hatched. The figures 

 exactly represent the two positions 

 that the inclined orifice will oc- 

 cupy when the right and left hand 

 portions of the object-glass are stopped off It was illumi- 

 nated as an opaque object, and drawn under a two-thirds 

 object-glass of about 28° of aperture. If this experiment 

 is repeated, by holding the card over the eye-piece, and 

 stopping off alternately the right and left half of the 

 ultimate emergent pencil, exactly the same changes and 

 appearances will be observed in the object under view. 



I 



Fiar. 65. 



