?0 M. MELI.ONI ON THE IMMEDIATE TRAXBMISSIOX 



submitted to the rays emerging from £uiy one of our live screens, we 

 Bhall always find the same ratios between the different terms of each 

 series. 



As to the black and the green glasses, their changes of transmission 

 occur sometimes similar, sometimes contrary to those of tlie other plates. 

 We should not however be surprised at these irregularities, as the green 

 and black colours alter the natural diathermancy of the glass and give it 

 an aptitude to transmit quantities of lieat M'hich will be more or less con- 

 siderable in proportion as the rays issiiing from the different screens 

 possess themselves a diathermancj^ more or less analogous to that intro- 

 duced into the vitreous substance by these two colouring materials*. 



* In a note to the preceding Memoir (page S) I have said that, for the study of ca- 

 lorific radiations the thermoniiiltiplier is preferable to every former thermoscopic 

 apparatus. The great number of experiments that I have since performed by 

 means of that instrument have produced in my mind a thorough conviction of 

 the truth of that opinion. As there ai-e still many e.\perimental researches to be 

 made not only in that class of phsenomena, of the history of which we have 

 scarcely given an outline, but in every branch of the study of radiant heat, it is to 

 be wished, for the interests of science, that every investigator would furnish him- 

 self with a thermomultiplier. This appai'atus, in the state of perfection neces- 

 sary to ensure good observations, is imfortunately one of those which a person 

 cannot construct for himself until he has made several attempts which are at- 

 tended with a great loss of time, and which cannot succeed in many places for 

 want of the requisite means. For these reasons I have thought it advisable to 

 put some one in Paris in the way of supplying them to the public. There are 

 excellent ones to be had at M. F. Gourjon's, rue des Nonandieres, N° 2. The 

 description of the ingenious means employed by this able mechanic to give 

 to the instrument every improvement which 1 was desirous of having in- 

 troduced into it would occupy too much time. I shall therefore confine my- 

 self to the mention of the principal defects found in the first instruments of this 

 kind presented to the Academy of Sciences by M. Nobili and myself (at the 

 sitting of the 5th of September 1831j, but now laid aside for the improved 

 thermomultipliers constructed by M. Gourjon. 



In the first place the volume of the thermoelectric pile was too bulky, (being 

 from 36 to 40 centimetres square in section,) a circumstance which rendered it 

 impossible to operate on small pencils of calorific rays : in the next place the 

 galvanometer did not mark fractions lower than half a degree, and the magnetic 

 needles, instead of standing at the zero of the scale, settled sometimes to the 

 right and sometimes to the left at a particular distance for each galvanometer, 

 amounting in some instances to 10 degrees. In fine, the mountings being almost 

 all of wood the pieces became warped by the hygrometrical variations in the 

 atmosphere, and the instrument was rendered unserviceable. 



The thermomultipliers of M. Gourjon have thermoelectric piles the acting 

 surfaces of which are not larger than the section of a common thermometer (3 

 centimetres square). As to the galvanometers they are mounted entirely in 

 copper with the exception of the small pieces necessary for the purpose of iso- 

 lation : the minuteness of their indications extends to a fourth and even a sixth 

 part of a degree, and '.he needles, when at rest, stand exactly at the zero of the 

 scale. It is almost needless to add that with these improvements the instrument 

 lias lost nothing in sensibility. 



