OF THE MICROSCOPE. 53 



bined with very high angular aperture, and in this view we believe 

 that we are in accord with the majority of our best microscopists. 



Working Distance. This 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 in the hands of experienced microscopists, but 

 beginners should by all means avoid them. Objectives of very 

 high angular aperture have in general very short working dis- 

 tances, but there are great differences in this respect amongst 

 the products of different makers. Working distance does not 

 depend upon angular aperture alone. 



Immersion .Lenses. Objectives which require a drop of 

 liquid between the front lens and the covering-glass of the 

 object, are now familiar to most microscopists, and have come 

 into very general favor. The liquid employed serves two im- 

 portant purposes. In the first place, it partially extinguishes 

 two of the glass surfaces (the front surface of the objective and 

 the upper surface of the covering-glass) and thus it prevents, to 

 a considerable extent, the loss of light which always occurs at 

 these surfaces. In the second place, it decreases very slightly 

 the magnifying power, and lowers the angular aperture, but in- 

 creases the penetration and working distance; hence immer- 

 sion objectives do not require the same precision of adjustment 

 for thickness of cover that dry lenses need, and consequently 

 it is possible to produce non-adjusting lenses on the immersion 

 principle which are easily used and very effective. They are, 

 therefore, great favorites with most histologists. Some makers 

 have carried this principle so far, that the objectives con- 

 structed by them can be used only with liquids much more 

 dense than water pure glycerine for example. 



For ordinary work we prefer a dry lens, from the fact that it 

 is some trouble to apply the liquid and clean the lens, and also 

 the cover, and where a large number of observations are to be 



