136 THE AMEKICAN MONTHLY [July, 



made use of in the Wenham Binocular Prism and Paraboloid, and some other acces- 

 sories. Dispersion of light and the properties of prisms and lenses were explained, 

 also the similarity of convex and concave lenses to prisms united either by their bases 

 or apexes. This was illustrated by drawings. 



Spherical aberration was discussed, and its remedy by means of a diaphragm, with 

 which to cut out the extraneous rays, was mentioned ; but, as this causes a great loss 

 of light, it is by no means as good a remedy as that generally adopted — the use of a 

 second lense of different form — a minifying lense to correct a magnifying one. 

 Chromatic aberration was explained by means of diagrams illustrating dispersion 

 by prisms, and, as a matter of course, by lenses, the violet rays being refracted more 

 than the red, and the intermediate colors in the order found in the solar spectrum. A 

 flint-glass prism, as it disperses the rays more than one made of crown glass, can be 

 used to correct the chromatic dispersion of the latter, while at the same time it allows the 

 ray to be refracted out of a straight course, and the same is true of lenses made of 

 these glasses. Up to a recent date, however, the achromatism was not perfect, because 

 the lengths of the corresponding colors in the two spectra were not the same, and there 

 resulted a secondary' spectrum. The new glass with which we have lately become 

 somewhat familiar, however, has eliminated this to a remarkable degree, and has 

 rendered the images of objects seen through it almost absolutely colorless. The images 

 produced by lenses, both real and virtual, were explained by diagrams. The ne.xt sub- 

 ject was the compound microscope. The passage of the rays of hght through the 

 objective and eye-piece was shown, and also the advantage of the field lense. 



' Angular aperture ' was defined as the ' angular difference between the paths of the 

 most divergent rays which an objective can gather up and bring to a focus.' Dry and 

 immersion lenses received attention, and the advantage of the latter over the former 

 in working distance and use of oblique rays fully explained. Lest there be some mis- 

 apprehension as to the meaning of what is known as a lens of i8o° angular aperture, 

 when this is known to be a straight hne perpendicular to the line of vision, Mr. Gard- 

 ner explained the matter, and said that it was equivalent to a balsam angle of 82°. 

 After the reading of the paper, Messrs. Gardner and Smith showed by the aid of a 

 lantern the action of lenses of different shapes upon pencils of light, the phenomena 

 of refraction and total reflection. The ever beautiful and fascinating experiment of the 

 stream of falling water, which imprisons the light (sent into it) by total reflection from 

 the upper and under surfaces, closed the meeting. 



o 



March i, 1888. — Meeting held at the residence of Mr. J. L. Smith at West Orange. 

 Mr. Smith had arranged upon a table his fine Powell and Leland stand, with a number 

 of accessories and a drawer filled with slides. He proceeded to explain to the mem- 

 bers some of the methods in microscopic manipulation which he had found very use- 

 ful in laboratory work, and by means of which the best performance of objectives, 

 condensers, etc., could be obtained. 



He first explained his method of obtaining axial light with the plane or concave 

 mirror alone. This was done by putting the object upon the stage, placing the mirror 

 as nearly central as possible, throwing the light upon the object, which is in focus, of 

 course, and then racking the tube up and down and noting the shadows, which, when 

 the light is axial, would not be thrown to one side or the other, but remain central. 

 This was demonstrated with a slide of Arachnoidiscus. He also stated that when using 

 the mirror the best position for the lamp was on one side, and not in front of the stand. 

 If the observer be in the habit of using his left eye in looking through the tube, the 

 lamp should be on the left side of the latter, as the tube then shields the right eye from 

 the flame. This position is changed when the right eye is used for observation. This 

 side position of the lamp also permits the best sort of reflection from the mirror, and 

 prevents, to a great extent, the formation of double images (z. e., from the silvered back 

 of the mirror and from the surface of the glass). 



The next operation was that of centering the substage condenser to high powers. 

 This is done as follows : — First put on the high power, having upon the stage any 

 familiar object, such as a diatom, which could afterwards be readily recognized in the 

 field of a lower power objective, e. g., an inch. Bring the diatom into the centre of 

 the field of the high-power objective (say jV^O- Now remove the twelfth, and, with- 

 out touching the slide, put on the one-inch ; focus down to the slide, and note the posi- 

 tion of the diatom, which will almost certainly be a little off from the centre of the 

 field of the one-inch. (It is, however, still in the centre of the field of the xs^^^.) Now, 

 with the V still on, centre the condenser to the object by noticing the images of the 



