ZOOLOGY AND BOTANY, MICROSOPY, ETC. 645 



2. The wave-surface of the colour E, which in aud for itself has the 

 second least spherical aberration, and, compared with the brightest 

 colour, the least chromatic aberration, departs the most widely from the 

 ideal spherical surface of the brighest colour. 



3. The wave-surfaces of the two colours (C and 550 fx.fi), which for 

 peripheral rays have the least cross-sectional difference, deviate on the 

 periphery the second-furthest from one another. 



4. The wave-surfaces of the two colours (D and F), which have the 

 maximum cross-sectional difference for peripheral rays, combine on the 

 periphery. 



The following statement may also be enunciated : — 



5. Those wave-surfaces of the two colours, E and 550 //. /*., incline 

 the least to one another from the axis to the periphery, which in the 

 spectrum lie nearest to one another, and, for axial rays, have the least 

 cross-sectional difference. 



In support or the foregoing statements the so-called Gauss construc- 

 tion may be appropriately quoted. 



6. If the section-distances for axial and peripheral rays of two 

 colours are equally great, then most certainly are the light-paths 

 corresponding to one another from the two wave-surfaces to the image- 

 point not equally long ; for (a) the medial errors (zones) are in both 

 colours of different magnitude, and therefore also the final result at the 

 periphery, (b) The refracted rays of the two colours (direct illumina- 

 tion being pre-supposed) claim different zones (red becomes more 

 strongly refracted than blue). 



In another case the author examined a giant objective of over 50 cm. 

 diameter and over 10 cm. focal length. It warranted the following 

 statement (optical paradox). 



7. If combined zonal errors were half as great as the actual ones, 

 then the definition-brightness (excellency of image) would be half as 

 great as the reality ; if the zonal errors were even less, then, indeed, 

 would the image excellency be rapidly augmented. 



K. Strehl hopes that the time may come when no expensive telescope 

 or Microscope objective will be sold without having been submitted to a 

 diffraction theory test. In the case of telescope objectives this would 

 have to be done for each specimen ; but in the case of micro-objectives 

 of a given number, the test could be made once for all. Neither can it 

 be objected that the application of the diffraction theory would be too 

 difficult or too tedious. On the contrary, it is quite easy, and at most a 

 specimen would only require two days. 



In another journal the author has an article entitled, " Test of a 

 Microscope Objective," * in which he describes his methods and gives 

 full details of his results. 



lf(3) ^Illuminating- and other Apparatus. 



Locking Arrangement for Microscopical Demonstrations/!" — A. 

 Fischer has designed an arrangement, more particularly applicable to 



* Untersuchung eines Mikroskopobjektivea, Zeit. f. Instrumentenk., xxv. (1905) 

 pp. 3-10(1 fig.). 



t Zeitschr. wise. Mikrosk., xxii. (1905) pp. 100-4 (2 figs.). 



