368 H. G. MAY. 



and down and comes to rest on the same facets only in the same 

 focal plane. But it is impossible to count all facets without 

 large changes in the focus. In shifting the fly after a given area 

 has been counted it is evident that the same shift makes the 

 hair pass over more facets on a slanting area than on a hori- 

 zontal one, just as surveyors will pass over more surface in 

 surveying up or down a hill than on the level. But on the 

 rounded eye the hair usually passes over both a horizontal and a 

 slanting area at the same time. For that reason one can not 

 possibly shift so that the hair crosses the same number of facets 

 at every point on the eye. A cross-ruled ocular has little or 

 no advantage over a cross-hair, and a camera lucida is scarcely 

 worthy of consideration as it is difficult enough to keep it at the 

 proper place even on a perfectly flat field. 



The largest error, by far, is due to the fact that only the right 

 eye was counted. Zeleny and Mattoon reported that they found 

 the averages of a large number of counts to be the same for both 

 eyes. The author obtained the same results. But that does 

 not mean that both eyes in any given fly have the same number. 

 Normally the variation is not more than about I per cent from 

 the mean, but the author has found it as high as 5 per cent, 

 and in one case 10 per cent. One abnormal male was obtained 

 with 25 facets in the right eye and 146 in the left. It is obvious 

 that the error is greatest in the parents and the extremes of the 

 offspring as we are dealing in those cases with individual flies. 

 In parents the left eye was usually examined to see that it was 

 not abnormal, and actual counts were made in a large number of 

 cases. The abnormal male was mated to see if the unequal 

 condition would be inherited, but it died within twenty-four 

 hours without giving any offspring. 



SELECTIONS IN THE VESTIGIAL-WINGED STOCK. 



When the bottle that was to give rise to the VBa stock was 

 received, flies were transferred to two fresh stock bottles and 

 the offspring from these parents were used for making the 

 original selections. From October 31 to November 5 some 

 preliminary counts were made on flies that hatched in the original 

 bottle. Sixty-nine males gave a mean facet number of 115 and 

 seventy females gave a mean of 63. 



