122 ANmJAL REPORT SMITHSONIAN INSTITUTION, 194 



in depth and is invisible to the naked eye. Arising above the 

 chromosphere, but ordinarily connected to it, we find the solar promi- 

 nences, tlie subject of this lecture. The principal instrumentation of 

 the McMath-Hulbert. Observatory has been designed and built for the 

 purpose of studying these solar prominences and other related 

 chromospheric phenomena. 



At this point it would be well to trace briefly the development of 

 our knowledge of chromospheric phenomena, as each step forward 

 usually rests on the achievements of earlier workers in the field. 

 Because of the fact that the prominences are rarely high enough to 

 subtend an angle as much as 2 minutes of arc, it is not surprising 

 that they apparently were not seen by the early observers of eclipses. 

 In fact, the discovery of the prominences had to await the invention 

 of the telescope. That event occurred early in the seventeenth cen- 

 tury, and nearly 100 years later Tannyan, at Berne, Switzerland, 

 reported the chromosphere, which he for.nd by observing an eclipse. 

 Discovery of solar prominences followed shortly thereafter, as 

 Vassenius observed them at an eclipse during the year 1733. These 

 are the most important prephotographic discoveries relating to our 

 subject. It is, of course, impossible to overestimate the importance 

 of the photographic process to science, and to astronomy in par- 

 ticular. Hence we are not surprised that at the ex;lipse of July 18, 

 1860, photographs taken 6 minutes apart by Secchi and De la Rue 

 showed that the prominences were moving. Also, by this time the 

 spectroscope had become a new tool in the hands of the scientist, 

 and on August 18, 1868, Janssen observed the bright spectral lines 

 of the prominences at an eclipse. It is of interest to note that at 

 first the bright yellow line of helium was mistaken for sodium. The 

 next day, August 19, 1868, Janssen observed the prominences without 

 an eclipse, by means of a widely opened spectroscope slit. This was 

 indeed an important addition to the available methods of observing 

 the sun. But we should note that it was October 20 of the same year 

 that both Janssen and Lockyer announced this new method. 



The next step, and it is a very important step, was the invention 

 of the instrument called the spectroheliograph by an American, 

 George Ellery Hale, in 1892. This instrument appears to have been 

 evolved independently and concurrently b)^ Evershed and Deslan- 

 dres; perhaps Deslandres may have been about a year later than 

 Hale. Two more years were needed to perfect the spectroheliograph, 

 and for many years it was the principal instrument used for the 

 study of chromospheric phenomena. A few more dates, and our 

 chronology will be sufficiently complete for our purpose here. 



In 1931 McMath, at Lake Angelus, started to adapt his technique 

 of practically continuous photography to Hale's spectrohelioscope, 



