176 W. S. Bay ley — Actinolite Magnetite Schists 



There does not seem to be any reason why this process with 

 the aid of well known optical devices should not be applied to 

 ordinary photometric work, or to such as is carried on in the 

 interior of the observing telescope of a spectroscope. In 

 simple work with discs like that here presented, the flicker 

 photometer is necessarily on the model of a stair case, and the 

 transitions though small are always abrupt ; but when there 

 are other modes of regulating the illumination this would not 

 be the case, and the advantage of multiplying observations 

 would be more fully experienced. Some experiments for 

 dividing up even the interval between two discs have been 

 begun, with promising results, but are not yet finished. 



It is well known that gazing steadily at a flickering surface 

 is trying to the eyes, but in the operations above described no 

 inconvenience whatever from this source was experienced, 

 since if the flicker was strong a glance revealed the fact and 

 another disc was substituted ; the faint flickering which de- 

 mands more prolonged attention seems to be no more trying to 

 the eyes than ordinary optical work. 



It may be a matter of some interest in physiological optics 

 to know that the sensation called " flickering," is independent 

 of wave-length and connected with change in luminosity. 

 When two differently colored surfaces of the same luminosity 

 are successively presented to the eye as above described, no 

 shock is experienced, and the colors are seen to mingle in a soft 

 streaky way ; if observation is prolonged for some time, sub- 

 jective effects begin to manifest themselves especially with 

 quite low rates of rotation ; it is of course best not to study 

 these with undue curiosity. 



Columbia College, June 19th, 1893. 



Akt. XX Y. — Actinolite Magnetite Schists from the Mesabe 

 Iron Range, in N oriheastern Minnesota j by W. S. Bayley. 



Attention has been repeatedly called to the existence of 

 beds of amphibole schists associated with the ores in the older 

 iron regions of the Lake Superior District. Brooks,* Wich- 

 mannf and Wright;}: referred to those occurring in Wisconsin 

 under the name actinolite schists and anthophyllo-actinolite 

 schists, and Julien| under the name tremolite-schists. Those 



*T. B. Brooks: Geol. of Wisconsin, vol. iii, 1873-79, p. 525, 577, 578, etc. 

 \ A. Wichmann: lb., vol. iii, p. 639, etc. 

 % C. E. Wright : lb., vol. iii, p. 699, etc. 

 | A. A. Julien: lb., vol. iii, p. 226, etc. 



