170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



Lyot invented an entirely new type of solar telescope — the Lyot 

 coronagraph. It permits the unwanted light from the sun's face to 

 be excluded from the final viewing eyepiece or film of the telescope, 

 but allows the light from the corona to come through undiminished. 

 The photographs of solar phenomena illustrating this article are all 

 taken with the Lyot-type coronagraph of the High Altitude Observ- 

 atory of Harvard University and the University of Colorado at 

 Climax, Colo. Figure 1 illustrates the principles of the corona- 

 graph and explains in brief terms why it works. The coronagraph 

 is one of the most important new research instruments developed 

 during our century, and it ranks in importance with the classic work 

 of Lockyer and Janssen. 



Today, scattered throughout the world, are coronagraphs of the 

 Lyot type. These permit their users to photograph prominences 

 and corona on all suitably clear days. I say "suitably" because the 

 standards of clarity are stringent. Coronagraphs have to be located 

 at high elevations in the mountains, and the slightest trace of atmos- 

 pheric dust or smoke blanks out their operation. 



But what of the results from the operation of the instruments de- 

 rived from the pioneering of Janssen and of Lyot ? The results make 

 up a major part of our knowledge of the sun and its variability. 

 Let me touch briefly on a few of these areas of knowledge. 



First of all, perhaps, in potential practical importance for man is 

 our growing knowledge of the influences of the sun on the weather. 

 At the Smithsonian Institution, for example, pioneering work has 

 long gone on. Other groups as well have devoted much effort to the 

 problem. Not only have C. G. Abbot, H. H. Clayton, H. C. Willett, 

 and others worked intensively on these problems, but the more recent 

 work of a man-and-wife team, the Duells, has uncovered a most un- 

 expected direct relationship between certain outbursts of solar activ- 

 ity and the trend of barometric pressures at terrestrial stations. This 

 problem has just been made the subject of over a year of most inten- 

 sive study by Richard A. Craig, of the Harvard College Observatory, 

 working closely with us at the High Altitude Observatory. In spite 

 of the fact that the field of solar-ter-restrial correlation studies is one 

 filled with pitfalls, Craig seems to have demonstrated beyond ques- 

 tion the reality of the fact that solar activity tends to have a direct 

 and statistically significant effect on terrestrial atmospheric pressure 

 changes. 



We have long known, from the pioneering studies of A. E. Douglass 

 of Arizona, the relationship between tree-growth rings and sunspot 

 activity. In fact, the relationship now gives us one of the main meth- 

 ods of dating the wood of Indian pueblo ruins. The cause? Still 

 unknown. And, of course, the relation between sunspot activity 



