ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 77 



ulready an appreciable intensity, which in ordinaiy cases varies from 

 1-2 p.c. in pentane to 14 p.c. in nitric-oxide of the intensity in a 

 perpendicular direction. 



2. Helium is an outstanding case, polarizing far less perfectly than 

 any other gas. The measurements give an intensity in the parallel 

 component nearly half as great as that which is in the perpendicular 

 component. 



:^. Theory indicates that this is the ratio to be expected if the 

 vibration in the helium atom is limited to a direction fixed within the 

 atom, on the assumption, of course, of random orientation to the 

 exciting light. Such an atom is the antithesis of the spherical atom or 

 molecule which would give perfect polarization. 



4. The intensity of scattering by the different gasses tried varies as 

 the square of the refractivity within the limits of experimental error. 



5. Saturated vapours, even when very dense, show no increase of 

 scattering power beyond what the density would lead one to expect. If 

 molecular aggregates are formed they are not numerous enough to show 

 by this metliod. 



G. Liquid ether apparently scatters about six times less light than 

 the corresponding mass of ether vapour. J. E. B. 



C5) Microscopical Optics and Manipulation. 



National Physical Laboratory Report for the year 1917-1918. — 



That portion of the report dealing with Optical Glass Testing is of 

 interest to microscopists. 



The number of specimens of glass experimented with has risen 

 largely during the past year. The Auxiliary Apparatus used in 

 connexion with these tests, which was mentioned in the last report and 

 was shown at the Optical Society's Exhibition held in January, 1917, 

 has continued to give the greatest satisfaction. 



The tests are made on a Pulfrich Refractometer, and experience 

 shows that a properly constructed instrument of this type, used under 

 suitable conditions, gives entirely reliable and accurate results. During 

 the past year investigations of a comprehensive character, both experi- 

 mental and theoretical, have been made on various aspects of Rt-fracto- 

 metry and on the types of instruments suitable for high accuracy 

 in such measurements. As a result proposals for a new form of instru- 

 ment have been made, but this unfortunately cannot be obtained during 

 the War. 



There have been several requests during the past year for the 

 examination for optical properties of specimens of glass wliich were not 

 specially prepared for that examination. Presumably this refers to the 

 testing of pieces of glass which have neither of their surfaces polished, 

 and which, therefore, cannot be tested by the Refractometer metliod. 

 The tests in these cases are carried out by an immersion method. 

 The accuracy obtainable varies with different specimens, but it has 

 proved sufficient in all cases to enable the particular glass employed to 

 be identified. The accuracy is, of course, less than that which is 

 secured when suitably prepared specimens are tested. J. E. B. 



