396 Transactions of the Society. 



for distinct definition ; the effect of which is to limit the illu- 

 minating rays, and prevent the object being drowned in excess 

 of light. A pin-hole stop limits the illuminating pencil to 

 perhaps ten degrees. ... A Beck iris diaphragm attached below 

 this kind of condenser (l^-in. objective of ten degrees) gives every 

 degree of fineness required in the illuminating pencil. Great 

 advantage is also sometimes obtained by stopping off half the 

 rays." 



Next, in 1877, Dr. Edmunds experiments with scales mounted 

 upon the slip, illuminating them by an immersion paraboloid (a 

 device of Wenham's) and finds that the exclamation marks are 

 featherlets. He measures one 50V0 i^^- lo^g' ^^^ ^oooiy i^- wide 

 (this measurement of the %vidth is more than 100 p.c. too large).* 

 We are thus brought back to the resolution of John Quekett's time. 



In 1882 Stodder and Hitchcock find that the exclamation 

 marks are not spines, and in the following year Moore and 

 Hitchcock find that they are spines. 



In 1888 T. F. Smith proposed a new image with a large axial 

 cone, and white dot focus ; he says that the exclamation marks are 

 precisely like pins, and that with this illumination the surface of 

 the scale between the exclamation marks exhibits structure ; 

 oblique lines can be seen springing from the head of a pin and 

 running to the points of the pins just above them on either side. 



In 1892 Dr. Mercer says that the exclamation marks are not 

 spines, but corrugations ; but Vereker and Wright regard featherlets 

 as the correct image. 



In 1895 Letherby says that the exclamation marks are perfora- 

 tions, and in 1906 Dr. Stokes writes confirming Letherby's view of 

 the subject. 



This necessarily brief history of the Podura test is now brought 

 down from the time of its discovery to the present day. The litera- 

 ture on the subject is large, and not readily accessible, because 

 many important observations appear indexed under the name of 

 some condenser, or other piece of apparatus, and therefore elude 

 search. A bibliography is annexed, which may probably be 

 found useful. Papers on the structure of scales other than those 

 of Podura are included in the list, as a good deal may l)e learnt 

 from analogy. This history amply justifies the statement that the 

 Podura scale is the microscopist's enigma, for it proves that 

 microscopists, who have for eighty years patiently studied the scale, 

 are nevertheless not agreed as to its structure. 



Before concluding, I ask for a little space for some observations 

 of my own ; but first we must weigh the respective merits of the 



* This measurement, which was no doubt correctly performed, is of much 

 interest, as the object was measured with dark-ground illmnination, and no allow- 

 ance raade for antipoint ; this should be compared with the uncorrected measure- 

 ment of 57jJuo °i^ ^ bright ground (infra). The mean of the two uncorrected images 

 is sslTnj i^-i while the corrected measurement is 3^^^ in. 



