372 



NA TURE 



\Feb. 19, 1! 



Foucault's prism, about the same as in Ahrens's prism, and 

 much less than in Nicol's prism. 



(2) Only half the prism is made of Iceland spar, a material 

 which is becoming deplorably scarce and expensive (I question 

 if there is in England or elsewhere a piece of spar fit to make a 

 Nicol's prism of 5 cm. aperture). The saving, however, is not 

 so great as it appears, since the spar-prisms a and b are con- 

 structed on Wollaston's principle, and involve a certain waste of 

 material. 



(3) The combination is not quite free from distortion and 

 chromatic aberration (the latter being due, of course, to irra- 

 tionality of dispersion ; it is practically impossible to achromatise 

 spar with glass), but the imperfection is not serious enough to 

 interfere with its use for many optical purposes, especially as a 

 polariser. 



(4) In using it, a diaphragm should be placed in such a posi- 

 tion as to limit the entering cone of rays to 28°, since at a greater 

 angle (at any rate, on one side of the field) the ordinary rays are 

 not separated by total reflexion. 



Doubtless the prism may be improved upon by better authori- 

 ties than myself; but I think that the principle of using a 

 "double-image" prism to increase the divergence of the 

 ordinary and extraordinary rays before one of them is separated 

 by total reflexion is worth attention. 



Ahrens's polarising prism is certainly a remarkable one. I do 

 not think that a double-image prism has ever been previously 

 constructed in which the extraordinary ray emerges without 

 deviation, while the other ray is deviated to the extent of very 

 nearly 6o°. H. G. Madan 



Eton College, February 17 



THE RESULTS OF THE SCIENTIFIC 

 EXPEDITION TO SODANKYLA 

 '"THE Government of Finland having provided further funds, 

 the Expedition has continued its researches at Sodankyla, 

 in Finnish Lapland, during the year 18S3-84 (Nature, 

 vol. xxvii. pp. 322 and 389). The plan of working this year 

 was chiefly confined to the study of the terrestrial galvanic 

 currents, atmospheric electric currents, and the phenomena of 

 light produced by the apparatus constructed by me for the 

 purpose. The number of daily meteorological and magnetic 

 observations was restricted to three, viz. at 6 a.m., 2 and 

 10 o'clock p.m., Gdttingen mean time, but on the rst and 15th 

 of each month observations were taken every five minutes, as in 

 the previous year, and on the 8th and 22nd of each month, from 

 8.30 p.m. till 10.30 p.m., observations were taken every half 

 minute. 



The general meteorological and magnetic observations were 

 continued without interruption until August 22, 1884. In the 

 account of the observations on the luminous phenomena will be 

 included a risitml of the general character of the weather of this 

 year. 



The Terrestrial Current. — From the middle of September 

 1882 the Expedition has observed the terrestrial currents, as well 

 as the magnetic variations. For this purpose two circuits about 

 5 km. long were placed north-south and east-west. They were 

 connected to platina plates 1 decimetre square, and buried about 

 I *3 m. below the surface of the ground. The southern and 

 eastern plates were about o'5 km. from the station. During this 

 year the observations were confined chiefly to the variations of 

 the terrestrial current, hence no attempt was made to separate 

 the electromotive force of the terrestrial current from that which 

 was developed by the contact of the plates with the earth. 



In the autumn of 1SS3 it became necessary to place fresh 

 wires in the circuits, and at the same time the position of the 

 plates was changed, so that each one was now about 2'S km. 

 from the station. The old circuit lying east and west was, how- 

 ever, left undisturbed for some time for the purpose of making 

 comparisons. 



It was not until the middle of January that observations of the 

 terrestrial currents were commenced at the auxiliary station at Kul- 

 tala, 68° 29' '5 N. (see Fig. I). Here the circuits for the terrestrial 

 current were placed so that the one lying north-south, 2'9Si4 km. 

 long, was 3" west, and the east-west circuit, 4*5663 km. long, lay 

 69° north-west. This arrangement was made to permit the 

 plates of the east and west being placed in the River Ivalo, and 

 those lying north-south, in two affluents of this river. At this 

 station attempts were made to eliminate that portion of the 

 electromotive force which arose from the contact of the plates 



with " earth " (here the water) as well as the polarisation. 

 The method employed was as follows : — With a Mascart 

 electrometer, the sensitiveness of which had been exactly mea- 

 sured by a " Daniell " normal element (about 18 divisions of the 

 scale per volt), the electromotive force of all the four plates in 

 the earth was determined. These were then joined in six 

 different ways with a galvanometer, and the deviations noted. A 

 Daniell normal element, furnished with an adjustable resistance- 

 slide, was then placed in the circuit in a contrary direction to the 

 current, and the electromotive force was then reduced till the 

 deviation was = o. Thus the electromotive force of the different 

 plates was obtained free of polarisation by means of an elec- 

 trometer. 



To eliminate the electromotive force arising from the contact 



O Observatory 



r.— Plan of the po 



The 

 M Kultata Station 



on of the apparatus on 

 Mountains. 



the Pietarintunturi 



of the plates with the water, the latter were taken to the 

 station and sunk in the river close by. They were connected 

 with a wire from the circuits resting on Mascart insulators. 

 Their electromotive force was examined by means of an electro- 

 meter, which was discharged each time by a plate of zinc 

 amalgam sunk in the river. This experiment was also made in 

 another manner. All the plates were sunk in a bucket of water 

 resting on Mascart insulators and connected with the earth by a 

 copper wire. The two latter experiments gave very similar 

 results. When the platina plates had been examined in this 

 manner, they were placed in their former positions, after which 

 they were again examined both by the galvanometer and electro- 

 meter. The details of this experiment, as well as those of 

 others, must, however, be reserved for a special memoir. By 

 the above-mentioned means results are shown free from any 

 accidental disturbing influences. Some observations, though as 



