4(t0 



Tliougli this melliud gives a clear sur\e3- of llie shut ul' tlie inter- 

 ference fringes and e. g. shows immediately, that the siiift for red 

 light (Fig. 4) differs from that for violet (Fig. 5), it is not very fit 

 to obtain quantitative results. By a detailed investigation I found, 

 that tliu uncertainty of the measurements was greater than I had 

 expected from eye observations. A disadvantage of (his melhod 

 is lirsl, tiiai for the measurement of the negative we must once 

 point on an interference fringe and tiien on the two pieces of a 

 broken fringe. For spectral lines this does not matter mucli, but the 

 difficulty becomes greater for the more hazy interference fringes. It 

 is however an essential disadvantage of this method that pointings 

 cannot be made on corresponding points of the interference fringes. 

 Quite satisfying results 1 got with the third method, 

 concerning which 1 shall give some details. In the 

 focal plane of the telescope a system of wires as 

 is shown in fig. 4 was adjusted. There are three 

 vertical wires (and one horizontal wire), so that 

 we can always choose the best one as a fixed mark 

 '''S- *• and read along the horizontal wire. It is very 



improbable that the three wires are all badly situated with respect to 

 the interference fringes. Just behind the cross wires the photographic 

 plate is adjusted on a plate-liohler (ohich is mounted indepeiidenth/ of 

 the telescope with the cross luires. The photograpliic plate can be brought 

 in the right position and slidden to take successive photos without 

 touching the telescope. Examples of the obtained photos are repro- 

 duced on the Plate (Fig. la — 5b), 4 or 5 times enlarged. The photos 

 la and Ih, In and 1h etc. belong together. Comparing two such 

 photos the shift of the interference fringes is evident. The dis- 

 placement is also given on the Plate in parts of the distance between 

 two fringes. As mentioned al)ove the measurement was made along 

 the horizontal wii'e. 



The width of the interference tVinges can he chosen according 

 to the circumstances. [> gives the pressure of the water in kilograms 

 per cm", measured during the streaming of the water with a mano- 

 meter coupled to the main tube, just before it divides into two 

 less wide ones. The times of exposition for the making of the 

 negatives amounted between 3 and 5 minutes. It therefore sufficed 

 to read the jiressure of the water each 3() seconds. The mean of 

 these reailings was taken as the pressuie during the measurement. 

 The variations in the pressure most times amounted only to some 

 hundredth parts of a kilogram, if by accident (what happened very 



