428 Prof. J. C. F. Zollner on Solar Protuberances. 



containing* his description of an arrangement whereby the solar 

 protuberances are rendered readily visible by means of a tele- 

 cope of only 12 inches focal length combined with a spectroscope. 



Yours faithfully, 



W. G. Lettsom. 



The magnitude of the solar image in the refractor, or, in other 

 words, the focal length of the object-glass employed, plays a very 

 prominent part in the entire method of observing the protube- 

 rances. It follows directly from the theory developed in a former 

 part of this paper, that with one and the same spectroscope the 

 contrast between the protuberance and the general ground is 

 dependent upon the width of the slit alone. But now, as with a 

 constant width of the aperture a so much greater part of the pro- 

 tuberance is seen at once the smaller the image of the sun is, it 

 follows that we should endeavour to obtain the amplification of 

 the protuberances we wish to observe, not by means of the solar 

 image (that is to say, not by the employment of an object-glass 

 of great focal length), but rather, as much as possible, by means 

 of the lenses of the spectroscope; and this can be readily brought 

 about by having recourse to a collimator of short focal length 

 compared with that of the observing-telescope. Assuming, for 

 instance, that we have a refractor of 10 feet focal length to which 

 a spectroscope is adapted, the focal length of both object-glasses 

 being equal — if in this state of things it is necessary to open the 

 jaws of the slit 1 millim. in width to obtain at once a view of the 

 whole of a protuberance of a certain extent, the opening of the 

 slit might be reduced to one-tenth of its former width, provided 

 the image of the sun were ten times as small ; while the protu- 

 berance would still remain visible to its entire extent, and would 

 be seen in ten times as strong contrast relatively to the ground 

 of the spectrum ; while, in order to arrive at the same amount 

 of amplification of the protuberance in the field of view (an am- 

 plification that we have sacrificed by a diminution of the size of 

 the sun's image), all we have to do is to give the collimator a 

 focal length ten times as short as that of the spectroscope. 

 We should therefore, adhering always to the instance we have 

 selected, with the same optical amplification of the protuberance 

 and with the same system of prisms, obtain a ten times as good 

 effect by employing, instead of a refractor of 10 feet focal length, 

 an instrument of only 1 foot focal length, and by giving to the col- 

 limator a focal length of about 2 inches, and to the observing- 

 telescope a focal length of about 20 inches. 



The quality of the images, as far as it is dependent on the 

 system of lenses, would be very little affected thereby, inasmuch 



