Astronomical and Nautical Collections, 137 



servation, it is sufficient to say that it readily exhibits the 

 fringes which are formed by the light of a star of moderate 

 brightness when an opaque body is interspersed, and even 

 divides the shadow of a wire into light and dark stripes, 

 when it is sufficiently fine, and far enough from the specta- 

 tor ; while even the shadow of such a wire would be imper- 

 ceptible to the nicest eye if it were simply projected on a 

 card. In order to see the fringes in the light of a star, it is 

 necessary to employ a lens of a pretty long focus, such as a 

 common spectacle-glass, of a focal length of a foot or two, 

 because, if the glass were more convex, the light would be 

 too much weakened ; but in this case the magnifying power 

 is not great, and it is impossible to observe very narrow 

 fringes in this manner. The glass, as already observed, 

 should be held for this experiment at such a distance from 

 the eye as to appear wholly illuminated, and to throw all the 

 light that it receives into the pupil. 



In order to measure the breadth of the fringes, that is to 

 say, the distances between the middle of the light and dark 

 stripes, it is sufficient to employ a small moveable lens, car- 

 rying a fine wire in its focus, which serves as a point of com- 

 parison or index, and the displacement of which may be 

 measured by a vernier, or by a screw, constituting the ap- 

 paratus which is commonly called a micrometer. That which 

 I have employed in all my experiments, and which was exe- 

 cuted by Mr. Fortin, is provided with a plate of brass, which 

 slides in two fixed grooves : this plate is perforated in the 

 middle by a hole -^^ of an inch in diameter, to the edge of 

 which is fixed on one side a single silkworm's thread, to 

 serve as a mark, and on the other a little tube carrying 

 the lens, which may be moved towards or from the thread 

 till the proper focus is found. The plate, on which these 

 pieces are fixed, is moved by a very accurate micrometer- 

 screw: the breadth of the threads is exactly ascertained, and 

 each revolution is divided by an index into 100 parts, so that 

 in this manner the change of place of the mark may be mea- 

 sured within -j^Vjy of an inch. It will now be easy to 

 understand how we can measure, for example, the distance 

 between two dark stripes : we bring the thread in succession 



