September, 1910. 



KNOWLEDGE. 



335 



Here is variety enough, in all conscience, to bear 

 testimony to the puzzle of this scale. \\'e find the 

 strange anomaly in its condition, that while contri- 

 buting so much as a test to the elucidation of 



objects, it yet 



structure in almost all other 

 persistently keeps the secret of 

 its own intact. A bold man in- 

 deed would he be, who says that 

 he knows the scale in every point. 

 In the earliest days, illustrated 

 by the first two examples in Mr. 

 Nelson's list, nothing seems to 

 have been made out beyond longi- 

 tudinal and oblique lines, and these 

 only to be seen by oblique light. 

 Andrew Pritchard, writing in The 

 Microscopic Cabinet, dated 1832, 

 savs : " I have never been able to 

 see the lines on them with a 

 power much below three hundred 

 and fifty (that is, one thirty-fifth 

 of an inch focus), and therefore 

 microscopes of a lower power 

 cannot be expected to show 

 them, except of a very superior 

 quality ; for it must constantly 

 be kept in mind that the instru- 

 ment is best which exhibits an 

 object with the least amplifica- 

 tion, all other things being 

 equal. . . . But they are most 

 easily made out, by the simple 

 light of a candle, in the aplanatic engiscope 

 (microscope) if it possesses an angle of aperture of 

 about fift\" degrees (about 43, N.A.) exhibiting all 

 their delicate minutiae with precision." 



Nothing can prove of greater interest to the 

 student of historical microscopy than comparing 

 illustrations in the infancy 

 of the science with \\hat we 

 know now. That the lines 

 on the Podura scale could 

 onl}- be seen under a power 

 of three hundred and fifty 

 diameters is a statement as- 

 tonishing until we remember 

 that it was only with an 

 objective of one-fifth inch focus 

 that the\- were able to obtain 

 a sufficient aperture to resolve 

 them. The same can now 

 be seen with an inch and a 

 power of one hundred. By 

 the time of Ouekett's first 

 edition of his Treatise on 

 the Microscope, dated 1848, the 



Figure 4. 



A Ruptured Podura Scale, first photo- 

 graphed at 1750 diameters, and 

 further enlarged to 3500. 



Figure 5. 



A Podura Scale, isolated "pins," magnified 1750 

 times. 



Ouekett's time deep eye-piecing was not to be 

 depended upon. 



So it remained until 1869, when Dr. Royston 

 Pigott startled the microscopical world by his 

 celebrated theory of the beaded structure of the scale. 

 Here the " exclamation marks " 

 had disappeared altogether, and in 

 their places were rouleaus of beads 

 crossing each other at oblique 

 angles, on different levels ; the 

 upper ones dark and the lower 

 lighter in colour. That is, accord- 

 ing to Dr. Pigott, who in illumin- 

 ating his object violated ever}- 

 principle we now think necessar\- 

 to produce a truthful image. 

 According to Mr. Browning, who 

 had seen the method, no condenser 

 w as used : the light was from a 

 common paraffin lamp ; the object 

 was seen under a verv deep eye- 

 piece, and a draw-tube extended 

 to sixteen inches. The most 

 astonishing thing is that any 

 microscopist in his senses should 

 have accepted the theory. Many 

 at that time, however, had 

 " beads " on the brain, and would 

 accept an\" theorv of structure with 

 beads in it, for any object. For 

 some five or six years the subject 

 was debated, for and against, as 

 can be seen in The Monthly Microscopical Journal. 

 Even Dr. \\'oodward threw his great weight into the 

 conflict, though more as an arbitrator than taking 

 sides : photographing the same scale, first with 

 illumination to show the " exclamation 

 with no trace of beading ; and then, with 

 oblique light and the focus 

 slightlv lifted, to produce beads. 

 It was open then to anyone to 

 take his choice. 



Quoting now from Mr. 

 Nelson's paper : " In 1888 

 T. F. Smith proposed a new 

 image with a large axial cone 

 and white dot focus ; he says 

 that the exclamation marks are 

 precisely like pins, and that 

 with this illumination the sur- 

 face of the scale between the 

 exclamation marks exhibits 

 structure ; oblique lines can 

 be seen springing from the 

 head of a pin and running 

 to the points of the pins 



central 

 marks." 



appearances seem to have developed into the now above them on either side.' 



familiar " exclamation marks." This, as far as the This fairly represents the appearance of the scale 



lower magnifications are concerned. It is difficult to as the writer knows it now, as photographed and 



explain that figured under a power of one thousand shown in the enlargement here, from the original 



two hundred and fiftv diameters, called by Mr. negative. On the other hand it is more than he had 



Nelson " featherlets," except by supposing that in discovered at the time of reading his first paper 



