1882.] 



MICKOSCOPICAL JOUENAL. 



53 



half a millionth of an inch — for if 

 not, then the lines of the plate must 

 run together. 



In regard to our first question, we 

 have already some evidence that Mr. 

 Fasoldt's assumption is not justified. 



Prof. W. A. Rogers ruled a 

 plate with his machine set for 500,000 

 lines to the inch, making every fifth 

 and tenth line longer than the rest. 

 He then measured the long lines, where 

 they projected from the band, and 

 found that they were so broad, that 

 they overlapped each other, leaving 

 no spaces between them. Evidently, 

 therefore, the band of 500,000 lin-es 

 did not consist of distinct lines. The 

 spectra were, nevertheless, clear and 

 bright. Hence, we are forced to con- 

 clude that the spectra do not prove 

 that Mr. Fasoldt's plate contains 

 1,000,000 lines to the inch. 

 o 



Illumination and Resolution. 

 — It is not unfrequently the case, as 

 we have had sufficient reason to know, 

 that when a person first tries to re- 

 solve the Amphipleura pelbicida, or 

 some other difficult test-object, with 

 an objective that is quite capable of 

 showing the lines, the result is very 

 unsatisfactory. Sometimes the lines 

 cannot be seen at all, and we have 

 known persons to own such objectives 

 for months, without being able to show 

 the lines. There is a " knack " about 

 it, to be sure, but it is by no means dif- 

 ficult to acquire. In most cases the 

 fault is entirely in the illumination. 

 An adjustable objective will not re- 

 solve well unless the proper adjust- 

 ment is made, and the lines on a deli- 

 cate test-object cannot be seen with- 

 out careful focussing — all this is well 

 understood at the beginning. Never- 

 theless, in most cases the greatest 

 difficulty met with by the novice is in 

 the management of the light. A few 

 suggestions concerning this matter 

 may prove of assistance to some of 

 our readers. 



One may sit at a table and put the 

 elbow on the partly opened drawer, 

 and hook the heel of the left foot over 



the front round of the chair, as Prof. 

 J. Edv/ards Smith has directed in his 

 celebrated book. But we can assure 

 the reader that lines on a A. pulliicida 

 can be reasonably well seen without 

 any such formalities. In truth, one 

 of the best resolutions we ever saw 

 was shown by Mr. Herbert Spencer, 

 in his shop at Geneva, with light from 

 a cloudy sky, the microscope stand- 

 ing on a packing-box, which was also 

 used as a seat, if we recollect aright. 



The novice will not succeed as well 

 by daylight as with light from a lamp, 

 and the simple, low hand-lamp with 

 a flat wick is the best of all. A stu- 

 dent-lamp is not good for this work. 

 To get the best result, remove all 

 substage accessories and also the 

 mirror. Then fasten the Woodward 

 prism, hemispherical lens, or what- 

 ever attachment may be used in their 

 stead, to the test-plate with glycerin. 

 It is a good plan to paste a strip of 

 paper on the back of the slide for the 

 side of the prism to rest against. This 

 prevents the prism from sliding down 

 if the glycerin does not hold it well. 

 The microscope should now be ar- 

 ranged so that the source of light will 

 be as nearly as possible at a right- 

 angle to the axis of the tube. If the 

 stand is a low one it may be raised 

 on a cigar box. Place the lamp about 

 a foot from the centre of the stage, 

 on the left, with the edge of the flame 

 toward the stage. Then introduce a 

 small condensing lens and focus the 

 flame carefully upon a piece of white 

 paper placed upon the stage. Then 

 put on the slide, with the prism attach- 

 ed, and it will not be difficult, by slight 

 changes in the position of the lamp 

 or bull's-eye, to make the lines 

 visible. 



It is much more easy to make a 

 resolution in this way than by using 

 the mirror. It is necessary to have a 

 sharp point of light upon the object 

 in order to get the best resolution, 

 and if the mirror is used it should be 

 carefully focussed upon the object. 

 However, a large mirror is very diffi- 

 cult to use in this kind of work, for 



