222 Prof. Draper's Description of the Tithonometer. 



From this it will be perceived that, taking the first experiment 

 as an example, if at the end of 30" the tithonometer has moved 

 7-00, at the end of 60" it has moved 800 more, at the end of 

 90", 7*50 more, at the end of 120", 7-75 more ; the numbers set 

 down in vertical column representing the amount of motion for 

 each thirty seconds. And, when it is recollected that the read- 

 ings 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 ex- 

 periments 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 ef- 

 fects 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 titho- 

 nometer 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 instru- 

 ment 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 be measured out. 

 The principle of the modification is easily apprehended. If half 

 the surface of the lens be screened by an opake body, as a piece 

 of blackened card-board, of course only half the quantity of rays 

 will pass which would have passed had the screen not been in- 

 terposed. If one fourth of the lens be left uncovered, only one 

 fourth of the quantity will pass ; but In all these instances the 

 focal image remains the same as before. By adjusting, therefore, 

 upon the wooden frame of the lens, two screens, the edges of 

 which pass through its centre, and are capable of rotation upon 

 that centre, we shall cut off all light when the screens are applied 

 edge to edge, we shall have 90° when they are rotated so as to 

 be at right angles, and 180° when they are superposed with their 

 edges parallel. Thus by setting them in different angular posi- 



