376 Rev. Mr. Pamirs Historical Sketch [May, 



and Christison found the photometer affected by simple heat : 

 hence Dr. Brewster concludes that no confidence can be placed 

 in the instrument. 



In Mr. Ritchie's additional observations (Edinb. Journ. v. p. 

 104), he ascribes Drs. Turner and Christison's result to using a 

 portable photometer, the heat rising to the top of the case, and 

 thus affecting the upper ball. He also states these further 

 objections ; the imperfect and unequal transparency of the case 

 causes the quantity of hght admitted to vary ; and the liaht 

 reflected from the inner surface varies at different altitude^ of 

 the sun. When the light is vertical (as in the torrid zone), the 

 upper ball will act as a screen to the lower ; in this case, also, 

 part of the light falling on the lower bulb is intercepted by that 

 part of it where it joins the stem, and thus excites heat. 



These objections appear to me to he well deserving attention. 

 I long since found that in using the instrument great differences 

 appeared in its indications, under different circumstances, which 

 must clearly have been owing to some of these interfering 

 causes. 



But there is another source of inaccuracy which deserves 

 peculiar notice. When light is absorbed by the black bulb, the 

 substance of the black glass in the first instance becomes heated, 

 and this before the heat is communicated to the enclosed air : 

 owing to this the glass expands ; and thus the first effect is 

 either an apparent contraction of the air, or a very diminished 

 expansion ; and at every successive instant, the glass continues 

 to receive more heat, to expand, and then to communicate heat 

 to the air; so that the effect is always retarded until the temper- 

 ature of the bulb becomes stationary by the balance of radiation, 

 when the temperature of the enclosed kir continues to rise for a 

 short time, till the equilibrium takes place in it also. The 

 expansion of the glass being now stationary, as well as the effect 

 on the air, the actual observed effect is the difference of expan- 

 sion of glass and air ; but each of these is proportional to the 

 degree of heat, and consequently their difference is so likewise. 

 Hence results obtained by taking the initial rate of rising with 

 this instrument are not comparable, except when the difterence 

 of intensity in the two cases is very small, or nothing. The 

 results obtained by observing the point at which it becomes 

 stationary can alone be relied on. 



Prof. Leslie proposed, among other applications of his photo- 

 meter, its employment to measure the effects of the different 

 rays of the prismatic spectrum ; in this case, the distinction is 

 sutficiently obvious that it applies only to the heating, and not 

 to the illuminating power of the rays. 



It may be matter of question, how far some of the objections 

 to this instrument, just adverted to, may show its unfitness for 

 giving accurate measures of the kind here proposed; there are, 



