452 M. E. Wilde on the Untenableness of 



could not be used for the purposes of measurement, and 

 hence I found myself compelled to resort to other principles 

 in the construction of my own. 



In the description which he has given of his gyreidoscope, 

 Jericau dissents from the theory of Newton's rings, which up 

 to the present time has been accepted ; inasmuch as experi- 

 ment has taught him, that, by pressing the glasses very 

 strongly together, the centre, instead of appearing dark, as 

 asserted by Newton, actually becomes bright. He endea- 

 voured to explain the matter by referring it to the repeated 

 diffusion of colours within a circle. I must however confess 

 that I am unable to attach any clear meaning to his explana- 

 tion. 



Prompted, however, by this remark of Jericau's, I was in- 

 duced to undertake a closer investigation of the subject as 

 soon as I found the gyreidometer in my possession. I have 

 not been able to corroborate the statement, that by pressing 

 the glasses very strongly together the centre becomes bright. 

 I have found that the first result of the pressure is a dark 

 central spot, which is formed at the summit of the convex 

 lens; but by increasing the compression and bringing the 

 glasses more closely into contact, the black spot expanded 

 into a circle with a bright centre; in the middle of this bright 

 patch a dark spot again appeared as the pressure was in- 

 creased, and it thus continued changing according as the 

 length of the paths of the respective rays reflected at the upper 

 and lower limit of the layer of air differed from each other by 

 an odd or even number of semi-undulations, until finally a 

 constant dark central spot was obtained, which it was impos- 

 sible to supersede even when the glasses were so forcibly 

 pressed, that the upper one, which was a quarter of an inch 

 in thickness, actually bent under the force. As the spot in- 

 creased in size, the surrounding curves diverged more and 

 more from the circular form. By ordinary daylight, as might 

 be expected, the change from dark to bright, which accom- 

 panied the approach of the glasses, could not be perceived ; 

 a variety of colours exhibited themselves one after the other, 

 until, finally, upon very strong pressure, the dark central spot 

 was obtained. A small number only of rings were visible when 

 viewed with common light; but with homogeneous light the 

 field of view of the microscope appeared covered with many 

 hundred curves, alternately dark and bright. 



But although I find myself thus unable to coincide with 

 the assertion of Jericau, a careful examination of the rings of 

 Newton with the gyreidometer has furnished me with other 

 grounds of dissent from the generally received theory which 



