556 Mr. H. T. Jessop on Cornus Mttlwd of 



In the experiments which follow, the distance between the 

 knife-edges was kept at 11*2 cm. Except in the case of 

 glasses 1 a and 2 this is more than three times the width or 

 the beams used, and the centre portion of the beams should 

 then be free from end-effects. The observations on glasses 1 a 

 and 2 were taken for the purpose of studying the effect of 

 the knife-edges on the change in transverse curvature. 



Observations of Strain Creep. 



When the first observations were taken by direct measure- 

 ment of the fringes, it was observed in some cases that a 

 second reading of the positions of fringes differed from the 

 first by an amount far exceeding the possible errors of 

 observation. This effect might have been due to any one of 

 three causes : — 



1. Movement of the microscope stand. 



2. A settling down of the cover-glass, with a consequent 



alteration in thickness of the film of air between it 

 and the beam. 



3. A change in the curvature of the beam. 



In any case it was impossible by direct measurement to 

 determine what was taking place, and accordingly photo- 

 graphs were taken of the fringes at intervals of time in order 

 that a thorough examination of the effect might be made. 



Figures 2 and 3 (Fls. XVIII. & XIX.) show photographs 

 obtained in four experiments. In each case the inner pair 

 of what are referred to in this paper as the ' transverse fringes ' 

 (i. e. the hyperbolas whose transverse axis lies across the beam) 

 have approached the centre of the system, while the ' longi- 

 tudinal fringes 3 have moved away from the centre. Photo- 

 graphs a, b and c in figure 2 show a pair of transverse fringes 

 crossing over to the longitudinal system. They are seen just 

 meeting in (a), just separating along the other axis in (5), and 

 more widely separated in (c). The same change has occurred 

 in the case of glass 3 (b) (fig. 2, c & d). 



In the case of glass 1 b (fig. 3; the fringes have not crossed 

 over. 



Some part of this change is undoubtedly due to settling 

 down of the cover-glass, as is shown in some cases (fig. 2, 

 b & <?) by the appearance of a new fringe at the edge of 

 the beam. This could not be avoided as any change in the 

 bending was bound to affect the state of contact of the 

 surfaces. The method of treating the measurements, however, 

 gave results for the curvature of the beam which were 

 independent of the distance apart of the surfaces, and by this 



