S7i Method of Increasing the Divergency of 



the long axis of the instrument in relation to the line of sight, 

 and when the power of seeing directly through is not at all dis- 

 turbed. In this manner the field of sight possesses dimensions 

 which are sufficiently great for optical experiments. 



Mr Nicol has not expressed any opinion as to the cause of 

 this great divergence. It is clear, however, that the chief cause 

 is to be sought in the action of the Canada balsam, which, ow- 

 ing to its refractive power (1,549), and which is that between the 

 ordinary (1,6543), and the extraordinary (1,4833), refraction 

 of calcareous spar, will change the direction of both the rays in 

 an opposite manner before they enter the' posterior prisma- 

 tic half of the combination. Without this action of the Cana- 

 da balsam, the cutting through and uniting the rhomboid would 

 be of no use, as was at first evident to us, but which has been 

 confirmed by experiment.* Whoever will take the trouble, 

 may calculate accurately the course of both rays, and even of 

 each colour, by means of the already existing data and formulae. 



Upon the plan just described, the mechanician Hirschmann 

 has prepared several of Nicol's rhomboids, which are in all re- 

 spects the same with that sent frgm England. Two of them 

 which I myself possess, are as perfect as I could wish them 

 to be. In looking through one of them at a fine line drawn 

 on paper, the line appears quite simple. -I* If we use both, and 

 place them behind one another, directing them upon an ob- 

 ject in a horizontal direction, we find, that, when the principal 

 sections are parallel, the object is seen with nearly as much 

 brightness and absence of colour, and with nearly half the dis- 

 tinctness, as when it is regarded with the naked eye ; if, however, 

 we turn one round until the principal sections are made perpen- 

 dicular to each other, we then have, at least in the middle of the 



• Mr Nicol was aware, that the increased separation of the images depended 

 on the Canada balsam with which the halves of the rhombic prism was united. 



If the two parts be united by water, the light sustains total reflection 



Edit. 



•\ On the other hand, the extraordinary image which is obtained by the 

 strong inclination of the rhomboid receives a mixture of the ordinary one, 

 and the field appears on that account brighter. This is of no disadvantage in 

 the use of the instrument. We also observe a series of feebly coloured edges 

 on the boundary of the two fields of sight, and this is also the case in the 

 arhomboids made in England. 



