794 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
correct as to focus, and would also copy a drawing with the marginal 
lines straight, that is, without any bending of the lines either out or in. 
He was urged to make a lens of this kind, and finally he did so, sending 
it to Prof. Sellers to test. That first lens, made as he had proposed, 
was perfect in its definition, and had all the good qualities he had 
promised. Most lenses for this kind of work have been the result of a 
long series of experiments ending in the form adopted. In this case a 
lens constructed upon a theory proved the correctness of that theory in 
a most remarkable manner. The Zentmayer lens, which in working is 
as rapid, if not more so, than other globe lenses in the market, was more 
simple, and filled a want, inasmuch as his system enabled him to make 
a series of lenses, the front of one lens being used as the back one of 
another through a series of sizes from the longest focus wanted to the 
shortest ; a set of these lenses, combined as required, meeting all cases 
that could occur both as to size of plate and proportion of reduction. 
Mr. Zentmayer’s patent for these lenses was not granted at once, but he 
was obliged to contest his claim before a master, in which on examination 
his claim in regard to priority was fully sustained. 
So radically original was the invention embodied in this lens, that 
the descriptions of it were at first regarded by the practical opticians of 
Europe as incredible, and as American exaggerations, and these ideas led 
to quite an animated controversy, which may be found in the ‘ Journal 
of the Franklin Institute,’ 1867, vol. lxxxiii. jj. 349 ; also 1868, 
vol. lxxxv. p. 153, and more fully in the ‘ Philadelphia Photographer,’ 
1867, vol. iv. pp. 177, 251, 253, 344 ; also 1868, vol. v. pp. 79, 109. 
After the system of screw-threads as the “United States” or the 
“Franklin Institute standard of screw-threads” was introduced, and 
makers called for instruments to measure the amount of reduction or 
the width of the flat top and bottom of the threads, a set of thin steel 
plates ground to an angle of 60° was sent to Mr. Zentmayer to have him 
grii:d the apex of each to the proper amount, the width of each being 
given to him in decimals of an inch to the fourth point. In topping these 
off he measured the flat by means of a stage and eye-piece micrometer. 
The correctness of his work was then verified by a member of this 
committee, who, taking the finished pieces, measured them on his own 
Microscope in the same manner, setting down the dimensions as found, 
and afterwards comparing them with what was required, with the result 
of finding them correct to the fourth place of decimals in each case. 
This was, in the first place, one of the earliest instances in which the 
Microscope was used in such a mechanical process, and a remarkable 
example of the facility with which good instruments can be used in such 
work of precision. The standard gauges, made since by the Brown and 
Sharpe Manufacturing Company, have all been adjusted to the standard 
pieces prepared by Mr. Zentmayer. 
The wonderful comparator designed by Prof. Rogers, of Boston, and 
made and used by the Pratt and Whitney Company, of Hartford, Conn., 
is furnished with Microscopes made by Mr. Zentmayer, who took great 
pains to perfect the instruments to be applied to this system of com- 
paring measurements. In all cases where work of great nicety has been 
required, those who knew Mr. Zentmayer’s skill were in the habit of 
seeking his aid, even in matters not pertaining to optics. The freedom 
