( 223 ) 



of the spoked disk, by which the ordinates are recorded, is very 

 constant. The error in the absolute amount may be estimated at no 

 more than 0,1 7o- ^^^^ tlie velocity of motion of the frame cannot 

 be so accurately regulated. The speed of a frame, once moving, may 

 be considered perfectly constant, but between photograms, taken at 

 various times, the speed may differ fairly much. Hence the distances 

 between the ordinates in one plate are somewhat greater than in 

 another. 



The distance between the abscissae is absolutely the same in all 

 plates. We could not expect otherwise, since the abscissae are recorded 

 b}' the shadow of a glass scale; mounted in a fixed position at a 

 few millimetres, distance from the movable frame. The glass scale, 

 containing very fine division lines, was expressly made for our pur- 

 pose by the firm Zeiss. 



The mutal distances of the abscissae and also of the ordinates are 

 measured under the microscope by means of the millimetre divisions 

 with nonius, which are found on the above described cross table. 

 For 30 scale divisions along an ordinate we invariably find 30,1 mm. 

 so that 1 scale division along an ordinate may always be put 

 = -J ,0033 mm. 



The scale divisions along an absciss in the middle half or middle 

 third part of the same plate are also equal, as may be seen e, g. 

 from the following measurements of photogram A 34, see table II. 



It must be remembered that the nonius did not enable us to read 

 with a greater accuracy than 0,1 or 0,05 mm. so that for these 

 comparatively rough measurements the mutual distances of the 



TABLE II. 



