PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 559 



We will now pass on to mention some experiments that were made gradually, first with three 

 round holes in a horizontal screen, next with two slits and then with three, in the horizontal screen, as 

 it appeared that when a part of the above-mentioned marked line of precipitation belonging to A fell 

 across a slit, a second line of precipitation appeared, which was also almost a right line, at an angular 

 distance from the first of 110. When this second line of precipitation was also placed over a second 

 slit, the rays once more passed through the screen and formed a third line of precipitation, which was 

 also turned about 110 in relation to the second line of precipitation. 



These experiments were both troublesome and lengthy, for every time an alteration was to be 

 made, the bottom had of course to be taken out of the discharge-box, and after the alteration had been 

 effected, the glass box had once more to be exhausted for several days, with frequent discharges, as 

 already mentioned, before it was again in perfect order. 



Photographs 5 and 6 refer to an experiment in which 3 holes were bored in the horizontal screen, 

 with their centres situated radially, as the figures show. The experiment was made with a pressure of 

 about 0.002 mm., with a discharge-current of 23 milliamperes through the tube, and a magnetising 

 current of 8 amperes upon the terrella. The eastern hour-angle of the vertical screen was 240; and 

 the photographs were taken respectively 120 and 60 to the west of the screen. 



By this slight magnetisation, beautiful precipitation was obtained on the terrella, when the hole 

 farthest in on the screen was brought over the first line of precipitation. It was easy to prove by a 

 slight displacement, that it was the rays that came from above and passed through the hole, that formed 

 the precipitation on the south part of the terrella, and vice versa. 



It will be seen that the precipitation does not only fall upon the terrella, but continues in the 

 second line of precipitation across the horizontal screen. By the employment of n amperes, the rays 

 through the first hole formed the western part of this precipitation upon the terrella, while, if the rays 

 were allowed to pass through the second hole by a slight turn, so as to bring the first line of preci- 

 pitation over the second hole they formed the eastern part of the precipitation upon the terrella, with 

 a continuation in the second line of precipitation on the horizontal screen. In these figures, 5 and 6, 

 we see distinct shadows of the pegs that are fixed in the horizontal screen. Much can of course be 

 concluded from the directions of these shadows, with regard to the course of the rays; but as the same 

 thing comes out more distinctly in another manner in subsequent experiments, we shall here only make 

 a few remarks. The shadow of the peg that stands on the first line of precipitation is faint, but often 

 extends some distance, and is curved almost like an arc of a circle with its centre in the centre of the 

 terrella. The shadows of the three pegs standing close together point outwards, and are formed of rays 

 belonging to precipitation B, which, however, is not distinctly outlined in the figure. 



Photographs 7 and 8 are taken with the same pressure as before, with 20 milliamperes through 

 the discharge-tube, and with 16 amperes on the terrella. 



There are now two slits in the horizontal screen, which here too have not been given quite their 

 correct form and position in relation to the first and second lines of precipitation. The angular distance 

 between the slits is, as will be seen from the precipitation, somewhat too small; for the slits could only 

 be determined by successive approximations, as the second line of precipitation does not appear until 

 the first slit is correctly cut, and the third line of precipitation until the second slit is correctly placed. 

 In the next experiment with three slits in the horizontal screen, however, the position and shape of the 

 slits are correct. 



In this experiment, Nos. 7 and 8, both the second and the third precipitation came out distinctly upon 

 the terrella, but not so well as in fig. 68 in Section I. The photographs are taken from places with an 

 hour-angle of 90 and 270 respectively. Although the first line of precipitation lies on the west side of 

 the first slit, the second line of precipitation, it will be seen, falls a little to the east of the second slit. 



