ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 299 



which is a good specimen of the kind of evidence seriously put forward 

 as sufficient to upset fundamental laws of light. It will he seen that 

 Mr. Dudley was unable to resolve the 160,000, 170,000, and 180,000 

 band, but that " the 190,000 band came out sharp and clear. This was 

 all he could do at that time ! " 



(1) Dudley's Report on the Examination of the Fasoldt Test-plates* — 

 Mr. P. H. Dudley reports his examination (on an invitation from 

 Mr. Fasoldt) of test-plates of his ruling, " as shown by his new vertical 

 illuminator, lamp, and specially constructed Microscope." It was, he 

 says, " an interesting and instructive evening." The stand was one con- 

 structed by Mr. Fasoldt, substituting a screw movement to the body 

 instead of the ordinary rack and pinion. The vertical illuminator had, 

 like Beck's, a thin glass for a reflector, but the method of mounting, 

 construction of the diaphragms, and means to control the light, were 

 "entirely different." The mechanical stage was constructed for the 

 purpose of making fine measurements, and comparing micrometers. 

 The eye-piece carried a micrometer, which had three delicate steel 

 prongs in lieu of cobwebs, or lines on glass. Each prong was adjustable, 

 extending partway across the field. One was in the ujjper part, and two 

 in the lower part of the field. The advantages of the prongs are many, 

 one being that but part of the line is covered. The lamp had a single 

 wick, 2 in. wide. In trimming, the wick was curved from edge to edge ; 

 the centre being fully 1/8 in. higher than the edges. The chimney was 

 specially formed of a metallic frame, carrying parallel plate-glass sides ; 

 those opposite the width of the frame about 3 by 4 in., and those opposite 

 the edges 3 by 2 in. On the top of the frame was put a metallic tube, 

 about 1^ in. diameter, and 14 in. high, to produce the draught. The 

 flame was large, and burned very white and steady. The lamp was 

 placed from two to four feet from the Microscope, the edge of the flame 

 being turned towards the illuminator, a small condenser, of 2 in. focus, 

 being placed before the illuminator, so as to throw an image of the flame 

 obliquely across the band of lines. The entire field was not equally 

 illuminated, as better results are obtained by having different portions 

 of different degrees of brightness. 



Photomicrograph No. 1 shown by Mr. Dudley was of a test-plate 

 having nineteen bands — said to have bands ranging from 5000 lines per 

 inch, to the eighteenth, which has 120,000 lines per inch. The nineteenth 

 band only has 50,000 lines per inch of the same depth of cutting as the 

 eighteenth band. These bands all having been resolved, new plates were 

 ruled, having finer bands. 



Photomicrograph No. 2 was of a test-plate with bands in the metric 

 measures. In one important respect the system of ruling on this plate 

 was modified. Each band, for a short portion of its length, was only 

 ruled with one-half of the number of lines in the rest of the band. 



Photomicrograph No. 3 was of a test-plate having twenty-three bands ; 

 the highest having, it is said, 200,000 lines per inch. The ruling was 

 very delicate, and the lines quite shallow, as must be the case. ' - Mr. 

 Fasoldt says twelve persons have seen the lines in the last band, 

 under his method of illumination, and with a Bausch and Lomb 1/12 in. 

 objective, N. A. 1 ■ 35." 



The first evening Mr. Dudley looked at the test-plates he saw the 



* Journ. New York Micr. Soc, iv. (1888) pp. 81-4. 



