NOTES AND MEMORANDA. 141 



accurate readings, and that it is to be regretted that it was not 

 arranged so as to read the angle in crown glass (i. e. the interior 

 angle) to degrees. It would have been far more convenient for 

 ordinary use, and just as easy to compute water, air, or glycerine 

 angles from the crown-glass angles as from the ordinary scale. 



Professor Abbe, writing to Mr. Stephenson, says that "for the 

 observation of bacteria the oil-immersion lenses are becoming more 

 and more appreciated by German microscopists. There is no doubt 

 your plan which enabled us to get rid of the refractions outside the 

 objective and at the front face, will be considered an important step 

 in the improvement of objectives. In addition to the increase of 

 aperture, the homogeneity of the medium from the object to the first 

 spherical surface turns out to be a great advantage in respect to fine 

 definitions." 



Diffraction Experiments with Plenrosigma angulatum. — Colonel 

 Woodward also says, in reference to these experiments,* that though 

 by lamplight he readily observed all the phenomena as described by 

 Professor Abbe, yet on trying by sunlight he obtained dificrent results. 

 The fine longitudinal lines produced by diffraction were distinctly 

 visible on all parts of all the frustules and entirely without limita- 

 tion to the adherent parts as required by Professor Abbe's theoretical 

 explanation. In the photographs of a frustule in which the adherent 

 parts are comjjaratively small (laid before the Society at the February 

 meeting), that obtained with the i showed the diffraction lines, after 

 the introduction of the diaphragm, on all parts of the frustule without 

 regard to the line of adhesion, while with the one taken with the jV, 

 the same was true for one side of the frustule, the other side being 

 slightly out of focus. A similar diffi-action picture of the right side 

 of the frustule could have been obtained, but then the left would have 

 been out of focus, a result of the form of the frustule. In neither 

 case are the diiraction lines limited to the adherent parts. When, 

 however, the illumination was obtained by lamplight the diffraction 

 lines were rigidly limited to the adherent parts. 



On these remarks Professor Abbe writes as follows : — 

 " The fact observed by Colonel Woodward that the longitudinal 

 lines on Angulatum appear throughout the whole frustule in observing 

 or photographing with direct sunlight, is not astonishing to me after 

 having considered the distance of those lines more accurately than I 

 had done before. The photographs give this distance (measured in 

 the middle part of both photographs) =0"335/x(fc = 0* 001 mm. = 

 1 micro-millimetre), the wave-length of D = 0*589 /a, F = 0'46/a. 

 Therefore the distance exceeds the half wave-length even of D, and 

 the lines are, theoretically, within the range of the numerical aperture 

 1 • for oblique light. It will thus be a matter of intensity of illu- 

 mination only, whether they will be visible or not visible through a 

 film of air, and it is quite natural that on the non-adhering parts of a 

 valve they are not visible with lamj)light, but yet are visible by 

 direct sunlight." 



* This Journal, vul. i. p. 53. 



