THE MICROSCOPE. 15 



and it was over a year before the glass was finished. It was, how- 

 ever, finally completed by Mr. Herbert Spencer. The objective has 

 four systems with the following diameters: 



Diameter of back system, - - -0733 of an inch. 



Diameter of middle system, - - - .0666 of an inch. 



Diameter of secondary front system, - .0444 of an inch. 



Diameter of front system, - - - .02 of an inch. 



The utilized diameter of the back lens is .065 of an inch. It 

 has ioo" balsam angle, equivalent to about 1.17 numerical aperture. 

 It is adjusted to work with either homogeneous immersion fluid, 

 glycerine or water. 



The total thickness of the four systems, from the front surface 

 of the front lens, to the back surface of the back lens is, .075 of an 

 inch. The working distance is about -.{j, of an inch. 



Placing the objective on one of Fasoldts test plates the observer 

 is astonished at the nice definition and flatness of field. The lines 

 are sharply in focus in every part of the field. 



With its use the ^. gemma is dotted and beaded elegantly. The 

 amphipleura pellucida is resolved very clearly. 



A private letter fron Prof. H. L. Smith, of Geneva, N. Y., says: 

 "The performance of this objective surprises me. Its work on the 

 podura is very fine, even up to 9000 diameters. I once had a -fa 

 made by an English house as well as one made by a celebrated Am- 

 erican optician. 1 need not say that this one of yours surpasses 

 them both in mechanical construction and in performance. It far 

 surpasses anything I have yet seen." 



Mr. Spencer says: "I believe it to be the best -fa ever made." 



I use this objective on a common stand, with only ordinary 

 means for illumination. It has been used at all times of the day, 

 and with many different kinds of light, yet never has it failed, both 

 to excite our admiration and to satisfy our expectations. 



With this superior glass I determined to do two things; by the 

 use of my cobweb eye piece micrometer, to more accurately measure 

 small objects; and also to settle in my own mind, if possible, the ques- 

 tion of "Heitzmann's reticulated structure of tissues." With what 

 success I have pursued, my investigations on various bioplasts, I 

 fully describe in this number of the journal. 



More may be said in favor of this objective, catalogued at 

 $270.00, in future numbers of The Microscope. 



