ZOOLOGY AND BOTANY, MICROSCOPY, KTC. 89 



MICROSCOPY. 

 A. Instruments, Accessories, etc. 



National Physical Laboratory Report, 1919. PLiblished by His 

 Majesty's Stationery Office. The following abstracts are of interest to 

 microscopists : — 



Optical Glass. — Dnring the latter part of the war the Laboratory 

 was called upon to test large numbers of specimens of glass for refrac- 

 tive index and dispersion. In connexion with this work a number of 

 investigations were made which proved to have important bearings on 

 other optical problems. The reliability of such measurements is of 

 great importance, as an error may easily lead to the waste of much 

 material, as well as of skilled lal)Our, in making systems which are only 

 found to be unusal)le when they are completed. Since the war the 

 demand for such tests has ceased, but it is very desirable that further 

 investigations contemplated in connexion with these measurements 

 should be continued. The tests are of fundamental importance to the 

 optical industry, and it is only the extensive experience acquired in 

 making them which has enabled the existing degree of accuracy to 

 be reached. One of the questions proposed for examination in this 

 connexion is the constancy of quartz and other natural transparent 

 minerals as regards their optical properties. Existing determinations 

 show variations which may either be instrumental errors or may be due 

 to variations w'ith different specimens. The source of these variations 

 can be traced quite definitely with the accuracy in measurement which 

 is now attainable, and it can thus be determined whether such materials 

 can l)e used with safety as standards of reference in the calibration or 

 checking of instruments intended for testing other materials. For this 

 purpose it is necessary to obtain a number of specimens of quartz wh( se 

 history is known. In particular it is desired to examine prisms of 

 quartz derived from all the chief sources at present known, j'hese 

 specimens should preferably be examined independently by a number of 

 investigators who have made accurate determinations of refractive 

 indices, with a view both to increased accuracy on the finally accepted 

 results and to the application of corrections for instrumental errors, if 

 such exist, to other measurements made previously. It is hoped that 

 specimens owned by other investigatoi's will be lent for examination at 

 this Laboratory. 



Optical Calculations. — For some years before the war extensi^'e re- 

 searches had been made into the methods employed in calculating optical 

 systems. It is hoped before long to })repare for publication a systematic 

 account of a number of the results already reached, which indicate that 

 methods of calculation differing from those in general use can be very 

 widely adopted with a great saving of time. Approximate methods of 



