OF THE MICROSCOPE. 77 



react many years ago but had forgotten, suggests that this be 

 called the penetration, of nigh angles in contradistinction to the 

 penetration of low angles as understood by Dr. Carpenter, 

 Prey, and others. Where the latter kind of penetration obtains, 

 all the layers are visible at once, as we have previously 

 explained. 



Working Distance. Considered as a quality of an ob- 

 jective, working distance is the distance between the front lens 

 and the object, and it is this distance, and this only, which can 

 guide us in forming an estimate of the quality of any given ob- 

 jective, as regards this particular feature. In practice, this 

 distance is diminished by the amount of metal used in setting, 

 which projects in front of the lenses, and also by the thickness 

 of the cover glass. It is evident, however, that these are mere 

 accidental circumstances which can be readily changed, and 

 which have nothing whatever to do with the quality of the ob- 

 jective as regards its optical characteristics. Some writers 

 have denned working distance to be the space between the 

 front of the lens and the upper surface of the cover glass ; but 

 it is very evident that if we accept this definition we can no 

 longer regard " working distance " as a quality of the objective, 

 since an objective which might be notable for its great working 

 distance with ordinary cover glasses, would have no working- 

 distance at all if very thick covers were used. 



To avoid this difficulty, the term frontal distance has been 

 introduced, and is used by some writers to signify the quality 

 heretofore recognized as working distance. But since the term 

 "working distance" has obtained a firm foothold in micro- 

 scopical literature, as expressing an important quality or feature 

 of objectives, we cannot see how any other meaning can be at- 

 tached to it than that given above. 



"Working distance is a very important feature in all lenses, 

 and good working distance is specially valuable to beginners. 

 There are many objectives in market that have to be brought 

 so close to the object that ordinary covering glass cannot be 

 used, and even with the thinnest glass, the distance between 

 the objective and the object is such that great skill and care are 

 required to avoid accidents. Such objectives do excellent work 



