REPORT OF COMMITTEE ON OBSERVATORIES 57 



could be advantageously employed at times. The use of such large 

 solar images should immediately permit advances of importance to 

 be made. But it would be wholly impossible to secure such images 

 with a refracting telescope of the ordinary type, for the correspond- 

 ing lengths of the telescope tube would be 215 feet or 322.5 feet, 

 requiring mountings of enormous dimensions, not only excessively 

 costly, but wholly beyond the possibilities of construction. For 

 this reason it is evident that another type of telescope must be em- 

 ployed if such large focal images are to be used. 



Fortunately it is possible to meet all the demands imposed by the 

 above named conditions. It is only necessary to mount the tele- 

 scope tube horizontally in a north and south direction and reflect 

 the light of the sun into the tube by means of a plane mirror driven 

 by clock-work. This mirror is the only moving part of the entire 

 mechanism. Its small size, as compared with a moving tube hun- 

 dreds of feet in length, not only renders the problem of following 

 the object a simple one, but it eliminates at once the entire question 

 of the enormous cost of the moving tube, the dome, and the great 

 elevating floor which, with an equatorial telescope, would be neces- 

 sary in order to permit the observer to reach the lower end of the 

 tube in all its various positions. 



Heliostats of various kinds have been used for many years, but until 

 recently no serious attempt has been made to construct a large hori- 

 zontal telescope. The instrument of this type built for the Paris ex- 

 position was never completed, and its design was such that the condi- 

 tions demanded in solar work could not have been met. The recently 

 completed Snow horizontal telescope of the Yerkes Observatory and 

 the horizontal telescope of the Smithsonian Astrophysical Observa- 

 tory represent the type of telescopes here recommended for solar 

 research. The Smithsonian instrument was designed for bolometric 

 work, and it has proved to be admirably adapted for this purpose. 

 The Snow telescope was designed for solar investigations requiring 

 a very sharply defined solar image, and special precautions were 

 taken to secure this result. Such a telescope accomplishes, at very 

 small expense, the purposes already named ; it brings a fixed image 

 of the sun into a laboratory, where it may be observed with large 

 spectroscopes or other apparatus mounted on piers. 



These instruments demonstrate the possibility of constructing a 



much larger telescope of the same type, designed especially for solar 



work and provided with large spectroscopes and spectroheliographs. 



To be successful such an instrument should be mounted at a consid- 



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