130 



THE AMERICAN MONTHLY 



[July. 



among many excellent observers there 

 are few who can handle a microscope 

 so neatly and quickly as Mr. Nelson. 



Entering the "studio" one after- 

 noon we found that daylight had been 

 effectually excluded, and several mi- 

 croscopes stood ready, with lamps 

 lighted, for inspection. The first ob- 

 ject we examined was a Heliopelta — 

 the beautiful diatom figured in Dr.Car- 

 penter's book — with a ^-inch objec- 

 tive. It was on a dark field, just as 

 we have often shown it ourselves, but 

 there was a peculiar beauty about it 

 which was soon explained. It was 

 illuminated with a Powell & Lealand 

 achromatic condenser, the light being 

 so adjusted that the most perfect 

 dark field effect was produced, in 

 combination with a polariscope and 

 selenite plate, which imparted just 

 the faintest tinge of blue to the 

 light. The diatom was not shown as 

 a polarizing object ; the polariscope 

 was used merely to color the light, 

 and thus impart a faint hint of color 

 to the frustule — so faint, indeed, 

 that it was just noticeable, but it was 

 enough to make the object stand out 

 with remarkable beauty and distinct- 

 ness. Some other diatoms shown in 

 the same way were also very fine ob- 

 jects for display. 



We may allude to the care with 

 which Mr. Nelson adjusted the light 

 for the dark field, not because there 

 is anything novel about it, but be- 

 cause our attention was particularly 

 drawn to the matter, and many per- 

 sons, perhaps, have not thought of it. 

 The source of light was a common 

 flame of an oil lamp, placed edgewise 

 to the mirror. In order that the ach- 

 romatic condenser should give a 

 round spot of light on the slide and 

 not an elongated image of the flame, 

 a bull's-eye lens was used in front of 

 the flame to render the rays as nearly 

 parallel as possible. To get the best 

 effect of illumination upon a minute 

 object like a diatom, the distance of 

 the source of light from the mirror 

 must be so adjusted that the spot of 

 light from the achromatic condenser 



focussed upon the object, will be as 

 nearly as possible of the size of the ob- 

 ject. No doubt this fact is often over- 

 looked in using condensers for dark- 

 field effects. Obviously the further the 

 light is removed, the smaller will be 

 the image formed by the condenser. 



The importance of careful illumina- 

 tion, in Mr. Nelson's opinion, is evi- 

 denced by the fact that he has a fine 

 adjustment attached to the substage 

 of his microscope for focussing the 

 condenser. Another refinement he 

 has adopted in his stand is to have the 

 eye-pieces of his Wenham binocular 

 stand, accurately adjusted so that 

 the images in both tubes shall be 

 magnified precisely alike. We doubt 

 if this is worthy of any special at- 

 tention. 



With a i^-inch objective by Powell 

 & Lealand, of numerical aperture 

 1.43 (balsam angle 140°) Mr. Nel- 

 son showed the longitudinal lines of 

 Amphiplcura pellucida so that we 

 were able to faintly distinguish them. 

 The resolution, however, was far from 

 satisfactory, and we are promised a 

 future opportunity of seeing the lines 

 under better conditions, when we are 

 assured that there will be no difficulty 

 in detecting them. 



The podura-scale was shown with 

 a -jV-inch, water-immersion of Zeiss, 

 with polarized light, but this object 

 has already been referred to in these 

 columns. A fine object for exhibi- 

 tion was a Pl.formosum, shown with 

 a Powell & Lealand |-inch objective 

 and lieberkuhn. The lens was of 95° 

 angle and had a very long working- 

 distance. The magnification was car- 

 ried up to 900 diameters. 



Of course we saw the transverse 

 lines of A. pcllncida with the fine 

 lenses in Mr. Nelson's hands. The 

 great advantage of the slightly in- 

 creased angular aperture of the ^V 

 inch of 1.43 n. a. over the ^^5 of 1.38 

 n. a., both by Powell & Lealand, is 

 strikingly shown by the resolution of 

 that diatom. 



We observe that the favorite method 

 of showing the lines on the A. pcUti- 



