"WITH REFERENCE TO THE POTATO DISEASE. 69 



roborates Sir John Herschel's statement regarding the variable- 

 ness of their respective effects: alluding to the negative effects 

 produced by the violet rays, he states {Philosophical Magazine, 

 vol. XXX. p. 90), " To my surprise I soon found that the negative 

 effect was gradually disappearing, and on September 29 it could 

 no longer be traced, except at the Idghest part corresponding to 

 the yellow and green rays. In December it had become still 

 more imperfect ; but on the I9th of the following March the 

 red and orange rays had recovered their original protective 

 power."; He adds, "Are there then periodic changes in the 

 nature of the sun's light ?" That this is the case seems highly 

 probable, from the vast changes which must take place in the 

 body of the sun or at his surface to produce those spots which 

 have of late been so conspicuous as to be seen in some instances 

 with the naked eye. In Sir John Herschel's Results of Astro- 

 nomical Observations made at the Cape, he mentions, as instances 

 of tlieir enormous magnitude, that one " occupied an area of 

 nearly five square minutes ; and as a square minute on the sun 

 corresponds to 756,000,000 square miles, we have here an area 

 of 3,780,000,000 square miles included in one vast region of 

 disturbance, and this requires to be increased for foresiiortening." 

 He mentions another which "would have allowed the globe of 

 the earth to drop through it, leaving 1000 miles clear of contact 

 on all sides of that tremendous gulf" 



In the present year, Mr. Pringle, of Edinburgh, gives an ac- 

 count of one of which, on the 21st of September, the diameter 

 was estimated at 60,000 miles. It would, therefore, occupy a 

 space more than fifty times the size which the earth would 

 require to drop through. The shadows of some of these spots 

 have been projected, and it has been found tiiat the shaded i)arts 

 have been considerably defective in heat compared with the 

 parts illuminated by the unobscured portions of the sun's disc. 

 These spots, it appears, interfere with the heating rays ; and 

 from the foregoing statements it is highly probable that the 

 derangements which have been observed to take place in tlie 

 chemical action of the prismatic rays are owing to the same 

 cau.>-e. In one instance Dr. Draper found the action of the red 

 ray inverted. May not similar inversions have been ^ufficient to 

 prevent the exhibition of the usual green colour observed since 

 the commencement of the disease in the young tips of the potato 

 stems, indicating an imperfect action of the light, and consequent 

 imperfect assimilation of the organic elements, diseased tissue 

 and secretions being the result? Whether those qualified to in- 

 vestigate this view of the subject will find these conjectures 

 correct or not remains to be seen. At all events, except in tlie 

 great source of light, I can find no cause sulficientiy univeisal 

 to correspond with the universality of the potato disease. 



