272 American Quarterly Microscopical Journal. 



Microscopical Society, November, 1878, p. 246), through which I 

 can throw the light at once at an angle of 45° by means of the 

 concave mirror or a small bull's-eye, and thus obtain for this 

 particular purpose equally good effects, with less expenditure of 

 time, in making the adjustments. 



DESCRIPTION OF A NEW APERTOMETER. 



HY J. J. WOODWARD, SURGEON AND BVT. LT. COL., U. S. A. 



(^Received J uiu' 4th, iSyg.) 



The interesting paper of my friend, Professor H. L. Smith, ac- 

 companying a description of his " Universal Apertometer " (this 

 journal, April, 1879, p. 194), suggests to me that your readers 

 would probably be interested by a description of the instrument I 

 have been using for some time for the same purpose. My aperto- 

 meter is really a combination of the apertometer of Professor 

 Abbe, of Jena, with the well-known sector, and has, I think, cer- 

 tain advantages over both the instrument of Abbe and the later 

 device of Professor Smith. 



Abbe's apertometer is essentially a modification of the instru- 

 ment described and figured by Mr. R. B. ToUes, of Boston, in 

 1873. ("An Apparatus for obtaining the 'Balsam' Angle of any 

 Objective." Monthly Microscopical Journal, Vol. IX., 1873, p. 212, 

 and Plate XV., lower portion.) This apparatus consists of a semi- 

 cylinder of crown-glass placed in front of the microscope ob- 

 jective, and the course of the most oblique rays, in this semicylin- 

 der, that can pass from it into the objective, or from the objective 

 into it, is measured by means of a shutter, sliding on its convex 

 surface. In practice, a thin glass cover is cemented with Canada 

 balsam over the center of the flat surface of the semicylinder, and 

 the objective is brought into optical contact with this by means of 

 the immersion fluid. The flame of a candle occupies the position 

 of the eye-piece of the microscope, and the light from this source, 

 after passing through the tube of the microscope and the objective, 

 diverges into the semicylinder in which the course of the extreme 

 rays can be measured by means of the shutter. Or, with the same 

 apparatus, if an eye-piece be used, and the objective focussed 

 ui)on a diatom or other transparent object mounted beneath the 

 thin glass cover, the course of the most oblique rays of light 

 thrown througli the semicylinder upon the diatom, with which 



