K S. Dana— Monasite from North Carolina. 247 



judgment of the distance and diameter of the combined image 

 was then recorded. Eight estimates for each of thirteen values 

 of the optic angle were made in irregular succession by each 

 observer, the experiments being distributed through two weeks. 

 I >n examination of the results attained, they were found not to 

 differ materially from those published in a former paper by 

 myself, as secured with continuous light. The limits of error 

 were much wider, as might be expected, showing that any 

 single judgment under such abnormal conditions has very 

 little value; but that, even when there is little opportunity for 

 play of the eyes, the effect of muscular strain on the internal 

 rectus and ciliary muscles is to modify the unconscious interpre- 

 tation of the retinal image, making the picture look much 

 smaller and nearer but not necessarily at the intersection of 

 visual lines. Mr. Share's judgments of distance and size were 

 almost uniformly a little less than my own. In cases of optic 

 divergence it was more difficult to secure the proper adjustment 

 of visual lines than in those of convergence. Distinct vision 

 was not attainable for divergence of more lhan —3°, though 

 with slight indistinctness I found it possible to attain the per- 

 ception of binocular relief for values as high as —7°. As a 

 limit, therefore, -3° was selected, and this was attained by 

 both observers, through voluntary control of the oeulo-motor 

 muscles. Since divergence of visual lines is never necessary in 

 ordinary vision, -i -h adaptation of the eyes, if these be normal 

 and not specially trained, requires usually two external points 

 of fixation, and time becomes an element of more importance 

 than when the coordination of muscular actions is such as habit 

 has made easy. 



Among the results of the miucrnlogieai investigations in 

 North Carolina, by Mr. W. E. Hidden, one of the most inter- 

 esting has been the discovery of the rare mineral mona/.ite at 

 a large number of localities. Mr. Hidden remarks* that at 

 Milli -Land's Mill, Alexander County, he has found, in a vein 

 in a ;_!-urnetiferous mica schist, a large number of monazite crys- 

 tals, together with fine geniculated crystals of rutile, quartz crys- 

 tals, pseudomorphs of limonite after siderite, and muscovite, 

 the last species making up the greater part of the vein. The 

 monazite was obtained by the concentration of the loose mate- 

 rial of the vein. Most of the crystals were very minute, of 



* This Journal. II f, x.vii, p V . 21, 22 r July. 1881. 



