14 Transactions of the Society. 



But in the microscope it appears quite the ten thousandth of 

 an inch, or nearly four times larger than it ought to be, if light had 

 no undulatory waves. And this too, whilst using the most exquisite 

 glasses obtainable. This tremendous fact shows how hopeless it is 

 to expect brilliant disks to appear of the proper or natural size, if I 

 may so speak, in the microscope. 



In view of the extraordinary result of the measurement of solar 

 spectra already alluded to, a very natural doubt will arise in the 

 minds of those who have not had practice in this method of minia- 

 ture, as to the correct effect of the glasses. Now the best process 

 for solving the doubt is to watch the spider threads successively 

 reduced from ten to fifty times. The operator will find it a slow 

 process, as every possible adjustment of centricity and correction for 

 aberration must be carefully attended to the whole time, as well as 

 arranging the light. It cost me at first about six hours' work. 

 But then the miniatures become so exquisitely smaller, the work in 

 its very novelty becomes fascinating, and encourages one to per- 

 severe. The observer will have no chance of splendidly defining the 

 millionth of an inch unless he is accustomed to high-power mani- 

 pulation, and remembers that both upper and lower objectives must 

 be both corrected by the screw collars for uncovered objects (dry 

 or immersion), and change of distance of the focal images. Some- 

 thing too should be understood of the efiect of change of " aperture " 

 upon the appearance of a transparent cylinder of spider silk. It 

 must be remembered that the aperture of the miniaturing objective, 

 as this is used in an inverted position, is greatly reduced as regards 

 the incident pencils emanating from the spider lines. 



A pencil of rays proceeding from the cross or intersection of 

 the spider lines about six inches i'rom the back glass, enters it at 

 an aperture of a few degrees only, perhaps ten. Now if a cylinder 

 of glass or spider gum be viewed with a low-aperture objective 

 (say H), it will present two black borders, and the breadth of these 

 borders narrows as the aperture is increased, and vice versa. Also 

 when the spider thread is diminished more and more, these black 

 borders appear almost to coalesce until only a black line appears. 

 The middle bright part vanishes with attenuation. 



Then it may be further urged that a very fine glass forms 

 miniatures of an object, theoretically, by merely optically reversing 

 the rays as perfectly, indeed more so, than in the enlarged image 

 of the same object. If therefore we can see the minute sjiider line 

 very perfectly magnified one thousand times, we can, a fortiori, see 

 the miniature, which is only fifty times smaller, with great precision. 

 So much for the objection against the accuracy of miniatures formed 

 by an excellently adjusted objective. 



But a crucial test is suj^'plied by observing sets of cross wires 

 separated a small space. Fortunately I had requested Mr. Browning 



