120 HOW TO SEE WITH THE MICROSCOPE. 



an infinite working distance, there would be no need of 

 special focal adjustment, and hence the longer the work- 

 ing distance the better. On the other hand, we are 

 told that objectives with very high apertures admit a 

 much wider cone of light, the lateral rays of which cross 

 in the focal point, at a more obtuse angle, and hence 

 the necessity of placing the object to be viewed exactly 

 in the focal plane. On all other planes, nearer or more 

 remote, the object being out of the crossing of the rays, 

 cannot be well defined; and here again, conversely the 

 presumed decrease of the working distance due to the 

 increase of aperture is held prominently in view. 



To all of the above, which has proved so acceptable 

 to the world of microscopists, the author long ago pub- 

 lished his dissent. He never did, and does not to-day, 

 take the least stocK. in the aforesaid enunciation of the 

 so-called doctrine of penetration. 



Admitting, as in the case of the two objectives pre- 

 sented, that the cone ot light illuminating the field 

 from the high-angled objective is wider, and that the 

 lateral pencils cross in. the focal point at a more obtuse 

 angle than can occur in the case of the narrow-angled 

 glass, it is nevertheless true (and singularly this little 

 fact seems to have been entirely lost sight of), that the 

 wider cone of light due to the employment of the wide 

 aperture includes all of the central pencils present in the 

 case of the narrow-angled glass. In other words, there 

 are just as many central pencils at work (and remember 

 that these are the fellows that cross the focal plane at 

 such an acute angle, thus furnishing the beloved pene- 



