402 Transactio7is of the Society. 



As a test for dry lenses, from N. A. ' 3 upwards, tlie Podura scale 

 is still -unrivalled. Differences between objectives can be detected 

 by a Podura scale, which no other test will reveal. It is to be 

 regretted that good test scales have become so rare and difficult 

 to obtain. Poduras are quite common, and are, moreover, easily 

 bred. Apart from its utility as a test, it is one of the most 

 beautiful of microscopical objects, whether illuminated by trans- 

 mitted or polarised light, or seen by dark ground illumination in 

 the manner described above. 



As the June No. of this Journal has been published before 

 this paper went to press, there is an opportunity of replying to the 

 criticisms made at the time it was read. 



1. If the blackness of the exclamation marks, when the vertical 

 illuminator is used, is due to Stokes' layer, the argument in the 

 paper still remains good, because a Stokes' layer cannot be formed 

 by a hole. 



2. If a Podura scale is placed between crossed Mcols, the 

 exclamation marks will become brilliantly illuminated, upon a dark 

 ground, at each 90° of rotation. 



If the Zeiss- Abbe test-plate is examined in the same manner, 

 the lines are almost invisible, however rotated. A very fortunate 

 accident has happened to one of my Abbe test-plates : a small 

 lilament has become entangled in tJoie quicksilver film, and the 

 ruling tool has cut through it, nearly at right angles. Now this 

 filament behaves precisely like the Podura scale, for it depolarises 

 the light and becomes brilliantly illuminated at each 90° of 

 rotation, and the contrast between the filament wliich depolarises 

 the light and the lines which do not is very striking. 



A Thorpe's impression grating, and a Grayson's dry-mounted 

 ruling of 15,000 lines to the inch, when rotated in a polarised field, 

 did not depolarise light. Diatoms, when mounted in balsam, give 

 very poor results ; the only one in IVioller's 400 type-slide to show 

 some illumination was the Eupodiscvs argus ; this remained illu- 

 minated at all azimuths, but no black cross was visible. An Actino- 

 ci/clus Ralfsii was very dimly illuminated, and four black dots 

 were noticed at the periphery, which may have been the extremities 

 of a black cross, otherwise invisible. 



Diatoms, when dry-mounted, gave far better results. Pleuro- 

 sigma formosum appeared much brighter, and upon rotation 

 .exhibited a certain amount of change. P. angulatitm was still 

 brighter, and exhibited marked changes at each 90° of rotation, 

 passing from a brownish-red to a pale greenish-yellow. P. halfi- 

 cum, however, gave the best results, and showed changes very 

 similar to those of Podiira. An iris-diaphragm was placed at the 

 back of the objective, and all the above phenomena were observed, 

 whether the diffraction spectra were admitted or whether shut out. 

 In order that there might be no mistake about this point, these 



