The Microscope. ' 325 



angle dry objectives of the Guiidlach Optical Co., tliat have 

 given most excellent satisfaction. Anxious to have the best, as 

 improvements were announced, I have ordered, from time to 

 time, five first class objectives, each one supposed at the time 

 to be the very best in the market. This paper may seem less 

 presumptuous with this statement inserted. 



SPECIFIC TESTS. 



I am now to report the results of some comparative tests, 

 made with certain named objectives, under described methods 

 of procedure. When the title of this paper was announced, I 

 hoped to have photographs taken in different ways for each ob- 

 jective tried, but have found too much time consumed in other 

 directions to permit it. Please allow me to express the convic- 

 tion, that these proposed photographs would have certainl}'- cor- 

 roborated the statements herein made. 



In order to decide, with certain correctness, of the relative 

 quality of the objectives compared, the tests were purposely made 

 as difficult as circumstances permitted, but under these difficulties 

 each was given the best handling possible for the manipulator. 

 The objectives, all homogeneous immersions, were as follows : 



Tolles' xVth, [1880] 123° ; Zeiss' apochromatic x^th, [1887] 

 N. A. 1.40; Herbert Spencer's xVth, [1888] balsam angle 180°; 

 Gundlach's Mh, [1890] balsam angle 186°. The last was asked 

 for and loaned to me for trial. An attempt was also made to 

 include a Leitz xVth, [1888] N. A. 1.25, but it was not possible 

 to use it on the same stand, hence not certainly under the same 

 conditions, and not included in this report. I have not been 

 able, under fairly similar conditions, to make it do what is re- 

 ported for the others. 



The first tests were made upon Mdller's balsam mounted test 

 plate, with an ordinary small coal oil lamp, with flat wick Hh inch 

 wide, and common round chimney, on which, however, was placed 

 a tin extension 16 inches long, to improve the combustion and 

 steady the flame. Anyone who sits down to a prolonged task of 

 this kind, will appreciate the latter, at least, of the improve- 

 ments thus obtained. The lamp was placed 80 inches from the 

 mirror to the left, with the centre of the flame used edgewise, 

 and mirror of the same height. The mirror bar was placed at the 

 angle of 50° from axis of the instrument, and the concave side 



