228 SUMMARY OF CURRENT RESEARCHES RELATING TO 



a vernier lens can be read to minutes. The graduation of the circle is 

 from 0° to 100° ; but there is a mark on the circle which must coincide 

 with the vernier-zero when the Microscope is to be vertical. The micro- 

 meter-screw M has a pitch of 0*5 mm. ; and its drum is, for dispersion 

 measurements, divided into 150°. 



Michelson Echelon Diffraction Grating.* — This apparatus consists 

 of a series of clear glass plates, each 10 mm. thick, overlapping in such 

 a way as to form a series of " steps " each 1 mm. wide. The plates are 

 all optically worked and should be in perfect optical contact ; there may 

 be fourteen or more mounted in a frame. A beam of parallel rays 

 transmitted through the series of plates is, therefore, retarded by 

 n t mm., where n is the number of plates and t their thickness. On 

 emergence the rays are in a condition to interfere. Though the echelon 

 can be used with almost any form of spectroscope, a special form known 

 as the " Constant Deviation Spectroscope " is the most convenient ; 

 the chief advantage of which is that neither collimator nor telescope 

 is ever moved, the echelon being rotated as required. Spectra of various 

 orders can be obtained in this way and are remarkable for their 

 brilliancy. Numerous practical details and other information are 

 furnished. 



Visibility of Ultra-Microscopic Particles.! — In the course of an 

 optical investigation of various shades of ruby glass, H. Siedentopf and 

 R. Zsigmondy devised a method of observing small particles of* gold 

 which closely approach molecular dimensions, and thus extending our 

 range of molecular vision very considerably. The ruby glasses, ex- 

 amined by the best ordinary Microscopes, appeared perfectly homo- 

 geneous. But the authors reasoned that if the gold particles imbedded 

 in the glass were at such distances apart that a Microscope could resolve 

 them, they could be made visible even though their size should be a 

 small fraction of the wave-length of visible light. The only condition 

 was that the product of the specific intensity into the surface of the 

 luminous particles and the square of the sine of the effective angle 

 of illumination should be greater than the inferior limit of the sensi- 

 tiveness of the human eye. The problem is thus reduced to that of 

 the visibility of a fixed star. What is seen is, of course, a diffraction 

 disc, and that is all we can hope to see, but the authors indicate a 

 means of determining the true size and weight of the particles seen. 



It is essential that all disturbing side-lights should be avoided. 

 The authors threw a beam of sunlight through a condenser on a slit 

 0'05 to 0*5 mm. wide, and an image of the slit was produced in the 

 field of vision by a telescope lens and a collimator with a reduction of 

 36 diameters. The diffraction discs seen in the ruby glass had an 

 average apparent diameter of 1 mm., while their real diameter, calcu- 

 lated from the quantity of gold present and the number of particles 

 counted in unit volume, was 0'02 p. on the average. This gives a 

 magnification of 50,000 diameters. The utmost limit to which the 

 magnification can be pushed by this method is about 150,000 diameters, 

 or 6 fifi. The average diameter of a molecule being ■ G /xf*, it cannot 



* Pamphlet by Adam Hilger, 75a Camden Road, N.W., July 1901. 



t Nature, Ixvii. (1903) p. 380. See Ann. d. Physik, No. 1 (1903) pp. 1-39. 



