240 Royal Society. 



Unfortunately I was prevented from making the use of the instru- 

 ment which I had intended at the display of meteors in November 

 1866. I have, however, great confidence in the suitability of the 

 apparatus for the prismatic observation of meteors and their trains. 



As the instrument is not provided with a slit, it is applicable 

 only to bright objects of small size, or to objects so distant as to 

 subtend but a very small angle. It is obvious that if the object has 

 a diameter smaller in one direction than in any other, as would 

 usually be the case with the trains of meteors, the instrument should 

 be rotated to take advantage of the form of the object. The most 

 favourable position will be when the smallest diameter of the object 

 is perpendicular to the height of the prisms. In this way I have 

 seen the lines of Fraunhofer in the spectrum of the moon when a 

 very narrow crescent. 



In the case of objects which appear as points, a small breadth may 

 be given to the spectrum by a cylindrical lens fitted in a little cap 

 which slips over the eye-lens, and is placed next to the eye. 



As some of the advantages which this instrument possesses over 

 an ordinary spectroscope, or over a prism held before the eye, may 

 be stated the comparatively large amount of light which the object 

 glass collects, the great facility for instantly pointing the instrument 

 to the object desired, which the large field of view affords, and in 

 some cases the magnifying-power of the instrument. 



It may perhaps be mentioned that secret signals might be con- 

 veyed at night by means of the temporary introduction of certain 

 suitable substances, as preparations of lithium, copper, strontium, 

 &c, into the flame of a lamp giving a continuous spectrum ; the 

 presence of the bright lines due to these substances would not be 

 perceived except by an observer provided with a spectrum-telescope, 

 to whom they might convey information in accordance with a pre- 

 vious arrangement. 



This little instrument, held in the hand and directed to the place 

 of the sun during its eclipse in 1868, might enable an observer, 

 who was not provided with larger apparatus, to give an answer to 

 the important question whether the bright prominences are self- 

 luminous or reflect solar light. At least it would be possible for 

 him to determine the general character of the spectrum of a bright 

 prominence so far as to learn whether it is continuous or consists of 

 bright lines. On account of the low magnifying-power of the in- 

 strument, the red prominence would appear sufficiently small to 

 permit of bright lines being distinguished on its spectrum, if such 

 should exist. 



The instrument should be previously focused by the observer on 

 the moon, or some distant object. 



Should a portion of the sun's limb be visible, the instrument 

 must be rotated until the spectrum of the little projecting prominence 

 appears in a direction parallel to that of the spectrum of the sun's 

 limb, and is not overlapped by it. Perhaps a diaphragm across the 

 field of view and cutting off about one-third of it would be an ad- 

 vantage, as the spectrum of the sun's limb might be concealed behind 



