266 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



If so, even with the very best lenses (except under special 

 conditions), light itself is of too coarse a nature to enable us 

 to define objects less than -^^-(ru to xooinru- of an inch apart. 

 It would appear, therefore, that, as far as this question is 

 concerned, our microscopes have already reached their ulti- 

 mate limit. Adopting the results as to the size of the ulti- 

 mate molecules of matter arrived at by Mr. Storey, Sir W. 

 Thomson, and Professor Clerk-Maxwell, Mr. Sorby calculates 

 that in the smallest interval which could be distinctly seen 

 by the best possible microscope there would be about 2000 

 molecules of liquid water lying end to end, or about 520 of 

 albumen, and that, in order to see the ultimate constitution 

 of organic bodies, we should require a magnifying jiower 

 from 500 to 2000 times greater than those we now possess. 

 He calculates that with our highest powers we are as far 

 from seeinoj the ultimate molecules of oro-anic substances 

 as we should be from seeing the contents of a newspaper 

 with the naked eye at the distance of a third of a mile. A 

 spherical particle one tenth the diameter of the smallest 

 speck that could be clearly defined with our best and high- 

 est powers might nevertheless contain no less than one mill- 

 ion structural molecules. 



sorby's apparatus for measuring bands in spectra. 



A plate of quartz one and a half inches in thickness is cut 

 so that the light will pass along the line of the principal 

 axis, and is interposed between two Nicol prisms. The 

 whole visible spectrum is thus apparently divided into eight 

 spaces by seven well-defined bands at equal intervals, and 

 with this peculiarity, that on rotating the polarizer or ana- 

 lyzer (the upper prism is furnished with a graduated ivory 

 circle) these black bands will move over the spectrum, oc- 

 cupying the original positions on completion of each half 

 revolution. Each band gradually passes from the red end 

 to the blue, in moving from zero to zero, over a semicircle. 

 Of course it becomes comparatively easy to construct a table 

 of wave-lengths, in millionths of a millimeter, corresponding 

 to each one -tenth division between the bands. The zero 

 point is determined by causing the upper Nicol to rotate 

 until the centre of the second dark band from the red end 

 of the spectrum exactly coincides with the sodium line, or 



