Professor Draper's Description of the Tithonometer. 405 



Table I. 



Showing that when the radiant source is constant, the amount 

 of movement in the tithonometer is directly proportional to 

 the times of exposure. 



From this it will be perceived that, taking the first experi- 

 ment as an example, if at the end of 30 seconds the tithono- 

 meter has moved 7*00, at the end of 60" it has moved 8*00 

 more, at the end of 90", 7'50 more, at the end of 120", 7*75 

 more; the numbers set down in the vertical column repre- 

 senting the amount of motion for each thirty seconds. And, 

 when it is recollected that the readings are all made with the 

 instrument in motion, the differences between the numbers do 

 not greatly exceed the possible errors of observation. It may 

 be remarked that the third and fourth experiments were made 

 with a different lamp. 



Though a certain amount of radiant heat from a source so 

 highly incandescent as that here used will pass the lens, its 

 effects can never be mistaken for those of the tithonic rays. 

 This is easily understood when we remember that the effect 

 of such transmitted heat would be to expand the gaseous 

 mixture, but the tithonic effect is to contract it. 



Next, I shall proceed to show that the indications of the 

 tithonometer are strictly proportional to the quantity of rays 

 that have impinged upon it ; a double quantity producing a 

 double effect, a triple quantity a threefold effect, &c. 



A slight modification in the arrangement (fig. 4) enables us 

 to prove this in a satisfactory way. The lens D, being mounted 

 in a square wooden frame, can easily be converted into an in- 

 strument for delivering at its focal point, where the sentient 

 tube is placed, measured quantities of the tithonic rays, and 

 thus becomes an invaluable auxiliary in those researches which 

 require known and predetermined quantities of tithonicity to 



