S42 The Microscope. 



I have recently had an experience with these substances that 

 has taught me, if not wisdom, at least caution in blaming my 

 objectives or even my own lack of manipulative skill. 



A certain homogeneous immersion objective, of not large 

 numerical aperture, was said to be able to resolve Amphipleura 

 pellucida well and easily. I made the attempt, and failed, after 

 several hours' work with the lens, using all the care and skill 

 that I possessed. The immersion fluid used had been prepared 

 and sold by a prominent optician, and I had no thought but 

 that its refractive index was what it should be. Another even- 

 ing was given to the examination of the lens over the same 

 Diatom; failure. A third evening was devoted to the same 

 work, and failure was the reward. I then gave it up, and con- 

 demned the objective or my own skill, being disposed toward 

 lack of confidence in the latter. Yet others had said that that 

 objective would resolve that Diatom. A fourth evening was 

 given to it with the same result. Then it suddenly seemed 

 stupid not to think to try another immersion fluid. There 

 might be something lacking in this. I had cedar oil from a 

 well-known European optician, and with a drop of it the objec- 

 tive was focussed, with the light as oblique and the mirror ex- 

 actly as before, when the lines on that shell stood out, if not 

 like the pickets on the fence, at least with a sharpness, clearness 

 and neatness that was as delightful as it was amazing. In the 

 twinkling of an eye the Diatom was resolved to perfection, 

 while with the glycerine fluid, failure and discouragement had 

 been the only results. The objective was vindicated and so was 

 any skill that the observer might, in a moment of self-compla- 

 cency, imagine to be his. But on the table were two other glyc- 

 erine fluids, one by a prominent and accomplished optician of 

 this country, the other by a famous American, who is by all 

 odds the equal of any optician in the world. The immersion 

 fluid from the latter refused to have anything to do with those 

 lines ; its action being similar to that of the composition first 

 tried. But the objective was not at fault, nor the adjustment. 



Tlie other fluid was then tried, and the resolution was in every 

 respect the equal of that made with cedar oil ; if anything it 

 was superior. But there was as usual the fly in the ointment. 

 To remove the glycerine from the objective it is necessary to 

 wash it off with water, but in this case, when the water drop was 



