lines crossing the Spectrum . 299 
fall in parallel directions on the prism, which is also proved by 
experiment. As the violet rays have by the object-glass of the 
telescope, though achromatic, a focal distance a little shorter 
than the red rays, we see clearly why it is necessary to displace 
the eye-glass, in order to perceive the lines distinctly in the dif- 
ferent colours. 
As the lines of the spectrum are extremely narrow, the apparatus 
must be very perfect, in order to avoid all aberration, by which 
the lines may be rendered indistinct, and even dispersed. The 
sides of the prism ought consequently to be perfectly plain, and 
the glass of which the prisms are made ought to have neither 
scratches nor strirn. With English flint-glass, which is never 
entirely free of these striae, we can only see the strongest lines. 
Common glass, and even the English crown-glass, contains many 
striae, though they are not always visible to the eye. Those 
who cannot procure a perfect prism of flint-glass, should use a 
fluid of great dispersive power, such as oil of anise-seeds, in or- 
der to see all the lines. In this case, the prismatic vessel ought 
to have its sides perfectly plane and parallel. In general, the 
sides of all the prisms should form an angle of 90° with their 
base, and this base ought to be placed horizontally before the 
telescope, if the axis of the telescope is horizontal. The narrow 
aperture by which the light passes ought to be exactly vertical. 
The reason why the lines become indistinct, if any of the con- 
ditions now mentioned is neglected, may now be readily under- 
stood. 
(To be continued .) 
Art. XVII . — Description of VeUie's Giel , a Scene in Ber- 
gen-Stift , in Norway . By the Rev. U. F. Borgesen * *. 
I had often heard of this remarkable Giel f , the only passage 
to a farm, considerable especially for the number of cattle reared 
on it. From the danger and the difficulty of the way, no clergy- 
man or other official person had ever visited it. What seems 
* Read before the Wernerian Natural History Society, 31st May 1823. 
*j* A Giel, in Norwegian, means a narrow glen with steep precipices on both 
sides, the space between filled up by a river. Vettie’s Giel is several Norwegian 
miles in length. 
