SUPPLEMENT. 363 



l-75th, having respectively the highest possible attainable aper- 

 ture. If this definition be accepted, then it occurs that what 

 was known as a high-angled l-10th of 130 ten years ago, would 

 now be classed as a medium-angled glass, and further, that au 

 inch objective of 45 would take rank as a high-angled glass, 

 and to the latter class of objectives do I refer as giving me the 

 best results for any and all work, selecting from this class of 

 objectives the one best adapted for the work in hand. Let me 

 illustrate by selecting a case that I almost daily have in practice 

 to- wit : 



Suppose, for instance, I am making preparations, say, of 

 malignant growths (notice that this is not a diatom), it would 

 doubtless be desirable tirst to make a preliminary examination. 

 I should, from the high-angled class of objectives, select the 

 above named inch of 45. Why ? 



First, because it has large working distance, which is not 

 disturbed by high eye-piecing. I am also enabled to bring the 

 mirror above the stage, and thus easily condense light on the 

 object ; and further, the working distance is so large that little 

 changes in the same do not disturb the object to such an object- 

 ionable extent as would occur had a higher power been selected. 



Second, because this inch will show me structure up to 30,- 

 000 lines to the inch. It will not only bear the l-4th solid eye- 

 piece, but will give with it added force of definition, with 

 increased amplification. 



Third, (result of first and second), because of all glasses I 

 have ever handled, I can get the most work (I have in hnnd) 

 out of it. For instance, its large reach, due to its working dis- 

 tance (the so-called penetration), enables me to focus through 

 the different planes of the object, while the entire specimen 

 remains fairly in view. I am further enabled to search through 

 my mount, with the least danger of allowing important struc- 

 tural elements to escape my attention an important item. 



.N"ow, should such preliminary examination suggest the pres- 

 ence of structure calling for an objective capable of recognizing 

 lines 100,000 to the inch ; the inch of 45 would be removed, and 

 a l-6th, having the widest aperture known, and working dis- 

 tance of one-fiftieth of an inch, substituted. With this supe- 



