122 THE MICROSCOPE. 



of a binocular instrument constructed by Mr. J. L. Riddell. This even 

 had many disadvantages and inconveniences, which Mr. F. H. Wenham 

 has very ingeniously succeeded in modifying and improving. 



In describing his improvements, he observes : " That in obtaining 

 binocularity with the compound achromatic microscope, in its com- 

 plete acting state, there are far greater practical difficulties to contend 

 against ; and which it is highly important to overcome, in order to cor- 

 rect 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 microscope ; then, with a card held 

 close to the object-glass, stop off alternately the right or left hand por- 

 tion of the front lens : it will be seen that during each alternate change 

 certain parts of the object will alter in their relative position. To 

 illustrate this, figs. 88 a 6 are enlarged draw- 

 ings of a portion of the egg of the common 

 bed-bug {Cmiex lecticula/ris), 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 in- 

 clined orifice will occupy when the right and left 

 hand portions of the object-glass are stopped 

 fig. 88. ofi'. It was illuminated 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. The two dif- 

 ferent images just produced are such as are required for obtaining 

 stereoscopic vision. It is therefore evident that if, instead of bringing 

 them confusedly together into one eye, we can separate them so as to 

 bring figs. 88 a 6 into the left and right eye, in the combined efiect of 

 the two projections we shall obtain all that is necessary to enable us to 



