100 Mr. J. J. Guest on the Strength of 



been practically completed; otherwise the alternative arrange- 

 ment of the twist-mirrors, which is free from such defects, 

 would probably have been preferred although not readily 

 applicable to specimens of different lengths. 



Bending in a plane through the telescope and specimen has 

 the effect of causing the image of the scale to move vertically 

 across the field of the telescope, but it does not affect the 

 reading. 



Bending in a plane at right angles to this, however, has an 

 effect upon the reading. If the angle CAP = ^and POA = ^, 

 we have 



cos 6= sin % cos ty, 



tan <}> = tan % sin ty, 



and bending alters the value of yjr, so that the effect of 

 bending is to produce 



~ n siny sin ylr -> , _ , 



86 = * . r o> = nearly. 



sin r J 



B(f> = 2j. -^ 8\fr=Syfr nearly. 



So that the bending is added to the twist. 



In the tests, however, a small load (250 lbs.) having been 

 placed on the specimen, the bearings were tapped until the 

 alignment had taken place. In pure torsion tests then the 

 bending could have had no effect; and in the combined tests, 

 as the torsion load was applied first and then the axial load, if 

 the latter produced any bending it would alter the reading of 

 the twist. On only one occasion was this of any magnitude, 

 and usually the reading remained nearly constant until the 

 yield-point was reached, when it increased. I therefore do 

 not think that the source of error indicated above had an 

 appreciable effect on the twist readings. 



38. As a Transmission Dynamometer. — It may be noticed 

 that in the case where the normals to the mirrors are perpen- 

 dicular to the specimen, the reflected optical axis is turned 

 through twice the angle the specimen twists through, and 

 this relationship is exact. By using a ray of light of small 

 cross section and allowing it, after reflexion at the two 

 mirrors, to fall upon a scale, its position will indicate the 

 twist between the mirrors ; and since the reading can be 

 taken when the light is intermittent, the device forms a cheap 

 and convenient form of transmission dynamometer. 



39. The Diametral Extensometer. — The diametral exteriso- 

 meter (fig. 14, p. 93) consists of a piece 1 resting in contact 

 with the specimen at the surfaces A B C D forming parallel Vs 



