SUPPLEMENT. 399 



chosen one of the highest balsam angles, in which case he will 

 have to fall back on his four-tenths. Nevertheless, let him, if 

 he will, try the experiment, and report as to whatever is gained 

 in point of penetration by the use of his one-forth of 50 over 

 and above what can be obtained by the use of the one-sixth, or 

 the four-tenths. The point I have to make is this the one- 

 fourth of 50 will have the greater penetration (so-called, like 

 the spectacle lens) but will lack definition, to the end that more is 

 lost in the latter element than is gained in the former. 



I state as a matter of fuct, but with no desire to bias the 

 further observations of Mr. Lapham, that I have used just such 

 a one-fourth as he describes, but have ultimately discarded it, 

 and in favor of a one-sixth marked by the maker 180 Q , which I 

 find will do all the work (penetration included) of the one-fourth 

 of 50 and a great deal more besides. But behind this one-sixth, 

 I hold a one-tenth of 100 balsam angle in reserve, for work 

 where " penetration" is ruled out. 



As to the other point suggested. Is there not undue weight 

 attached to this "penetration?" "With me, I have often been 

 obliged to take special measures to get rid of this function, and 

 for this purpose use the Beck illuminator, which gives me only 

 surface structures. I mention the fact, but am willing to admit 

 that a certain amount of - penetration" is at times desirable, 

 and should be provided for as perfectly as possible. 



Mr. Lapham recommends a four-tenths of 100. I have often 

 thought that such a glass, or a half-inch of the same angle, 

 would be desirable, and, as a luxury, arn still of the opinion. It 

 must be remembered that he very properly rules out the item 

 of cost ; wnile on the other hand I have made it a study to avoid 

 expense, where my opinion has been solicited in the matter of 

 selection of objectives. 



Mr. Lapham recommends a certain number of low-angled 

 glasses, all others to be of the highest angles, and of the best 

 quality and well corrected. Why not amend this by insisting 

 that all glasses have the highest attainable angles? selecting 

 of such the one suitable for the work in hand. 



Again, Mr. Lapham states that we have no need to consider 

 objectives of lower power than the half-inch for " they are not 



