May, 1913. 



KNOWLEDGE. 



189 



Table 37. 

 Results of first counting : — 



Nine ring systems in 39 corresponds to 23%. 



Thirteen coincidences in 41 counts to 31%. 

 There are, however, 13 coincidences, one off, so that we 

 have about li what chance should give. 



* If we count in the rays we have numbers from 55... 99, in 

 which series there are again 9 ring systems or 20%. Excluding 

 the ring systems under 52 we then have just 15 coincidences, 

 or about 34%. 



Table 38. 



Thus the total number of coincidences is 48. The range is 

 28 to 99 or 71 with 13 ring systems. 

 13 x 



71 



77^. = 18-3% ring systems. 



48 x 



-jrr = , 0( ; =52-8% coincidences with ring systems, or nearly 



three times what chance would give. 



If, however, we take the three commonest forms 17, IS, 19, 

 with the most numerous countings we get 17 range 35. ..82, 



i.e., in 47 digits there are 10 ring systems or 21 -25...% obser- 

 vation gives 55-5%. 



18 range 34. ..82, 48 digits with 10 ring systems or 20-8% 



observation gives 90 • 9%. 



19 range 44. ..90, 46 digits with 11 ring systems or 24-0% 



observation gives 57-8%. 

 21, 20-0% observation gives 50%. 



W. W. STRICKLAND. 



HEN BIRDS ASSUMING THE PLUMAGE OE THE 

 MALE BIRD. 



To the Editors of " Knowledge." 



Sirs. — The experiments carried out at Graaff Reinet, Cape 

 Colony, in removing the ovaries of the hen bird in Ostriches 

 and thereby causing the plumage to be that of the male bird, 

 are very interesting. 



As is well known, hen pheasants and probably hens of 

 poultry, also assume the garb of a male bird if the ovaries 

 become diseased, or are removed, and it would be interesting 

 to carry the experiment still further, and find whether all hen 

 birds would be affected in the same way. I think it is probable 

 that it would be so. 



Some years ago an old male Ostrich in these Gardens 

 assumed the plumage of a hen bird, and was evidently badly 

 diseased before the change occurred, as he shortly after died ; 

 unfortunately I was absent at the time, so the skin was not 

 preserved. 



The effect of the removal of ovaries in animal life is well 

 known. 



DUDLEY LE SOUEE. C.M.Z.S., Director. 



Zoological Gardens, 

 Melbourne. 



ON THE RELATIONSHIP OF APERTURE TO 



POWER IN MICROSCOPE OBJECTIVES. 



To the Editors of " Knowledge." 



Sirs, — After reading Mr. Hutton's interesting article on the 

 craze for aperture, which appeared in "Knowledge" 

 for February, I looked up some of the points in Wright's 

 " Principles of Microscopy " and came across the following, 

 which raised the question, " How are we amateurs to decide 

 when doctors disagree." On page 225 the following is 

 given : — 



" Example : In the case where a 100-fold magnification is to 

 be exacted . . . and where our choice lies between . 

 N.A.'s of - 6 and 0-3 respectively, preference ought, other 

 things being equal, to be given to the former. The 0-6 

 objective would not only gather in more light into the image 

 and give better resolution in depth, but it would give us the 

 required magnification in association with a terminal beam of 

 0-006 N.A., i.e., a beam which would fill the whole aperture of 

 the pupil. The 0-3 objective would, in association with a 

 hundredfold magnified image, give us a beam of 0'0003 N.A., 

 i.e., a beam whose diameter would be less by one half than 

 the diameter of the pupil." 



The differences between the two writers is curious. Mr. 

 Hutton, taking the average power of resolution of eyesight at 

 one hundred and twenty five lines to the inch, gets an N.A. of 

 0-13, sufficient with a magnification of one hundred. Sir 

 A. E. Wright, taking the diameter of the pupil, recommends 

 an N.A. of 0-6 for the same magnification. Thus, while both 

 are guided by the limits of human eyesight, the results they 

 arrive at are widely different. 



Nor is this the only difference between the two writers, for 

 Mr. Hutton says: "Besides, we sacrifice depth of focus" by 

 going in for a larger aperture, whereas, according to Sir A. E. 

 Wright, quoted above, the higher N.A. gives " better resolution 



in depth." 



Riverside, 



Castle Street, 



Salisbury. 



C. H. De MF.LLO. 



