INVKSTICATKI) BY THI. MKTHOD OF .TET VIBRATION. 371 



measured, we have X,. This would be perfectly correct if the jet were horizontal over 

 the whole length, in other words, parallel to T,-T,, and perpendicular to the light 

 incident on the jet. As this is not the case, the following correction is necessary : 

 L represents the distance hetween the homologous points in the image, hut what is in 

 reality necessary to l>e known is the distance between the corresponding points p and y 

 on the jet itself. With the symbols used in the figure, this correction for the point 



p will be with sufficient exactness : x = -,[ab + ac (ai 



Here e is the distance from the point on the image to the point Q (see fig. 17) 

 where the ray of minimum deflection is reflected. This correction is calculated for both 

 the two points p and </, and the distance L, Ixjtween these points is L, = L x x,. 



This formula is not quite correct, as e as a rule will have different values at the 

 two ends, but the corresponding error is only small, and will be neglected here. L, 

 is here determined as the distance between the points p and </, although in reality it 

 is the length of the portion /> 7 of the jet that is needed ; but this error is only 

 small for the jets examined here. 



The wave-length is therefore determined by X, = L,/^. 



In the following, the wave-length is always determined by the last-mentioned 

 method, although perhaps it is not so exact as the first, in principle ; it has never- 

 theless great advantages compared with it. Among these advantages is the 

 comprehensive view of the whole jet, tending to prevent mistakes, and the much 

 shorter time needed for the determination, inasmuch as the actual measuring work 

 can be done afterwards on the finished plate. Finally, the exactness that is reached is 

 certainly as great as is possible, so long as it is not feasible to obtain absolutely pure 

 jet-vibrations. One fault, however, with this method is that it is only available for 

 transparent liquids. 



1 hiring exposure the plate P is arranged in a plate-holder which is fixed in a 

 vertical frame. This can be laid down in a horizontal position by turning the pivots 

 below. The frame is arranged on a horizontal slide that can move in a direction at 

 right angles to the jet. The movement of the slide towards the jet is stopped by an 

 adjustable stop leaving a distance of about 4 mm. between the plate and the jet. 



< Mi Plates 3 and 4 are shown some photographs of jets taken in this manner ; further 

 details will l>e given later. 



By the use of nearly monochromatic illumination still tatter jet-images may be 

 obtained. 



Inr< xtiti'ition* on the Influence of tin- Amplitude of Vibration, 



;; 1 0. If the jet's cross-section is determined by the equation 



r = a + b cos nf> ....... . ., % . (1), 



then r nu , x = a + b and /,, = a b. 



3 B 2 



