in Solar and Oxyhydrogen Microscopes. 185 



of temperature is indicated by the thermometer at the foci of 

 these two lenses. It appears, however, upon trial, that the 

 crossing of the rays between the lenses so materially affects 

 the illuminating power, ^hen a terrestrial source of light is 

 used, that the applicability of this arrangement to the oxy- 

 hydrogen microscope becomes extremely questionable. I have 

 therefore proposed two new methods, which appear both to 

 improve the light and to remove all injurious heat. 



The light is improved by a combination of different lenses. 

 After the rays proceeding from a column of lime have been 

 rendered parallel, they are received on what may be properly 

 termed the condensing lens of the instrument. A very deep 

 double concave lens is then placed within the focus now ob- 

 tained, so that the condensed emergent pencil is of small dia- 

 meter, parallel, and nearly achromatic. We then proceed as 

 usual with the field-glass and the object-glass. With this 

 arrangement only have I been able to see, with entire satis- 

 faction, the longitudinal and cross lines on those scales of 

 butterflies' wings which are received as test objects-, and this 

 distinctness evidently arises from effecting, by means of the 

 concave lens, an almost entire removal of the blue rays, 

 which in any other adjustment of lenses occupy the middle 

 of the illuminated disc. 



The heat is also partially removed by this combination of 

 lenses ; for, as calorific rays, like luminous rays, are suscep- 

 tible of refraction, it will follow from the different positions of 

 the principal foci of light and heat in the axis of the conden- 

 sing lens, that when the concave lens renders the colorific rays 

 parallel, the calorific rays will, in this case, diverge. But to 

 procure a practically efficient removal of the heat, I have pro- 

 posed to transmit the rays through a current of air. That 

 such a method should not have occurred to Melloni cannot 

 but be a matter of surprise, though perhaps, indeed, it may 

 seem somewhat unphilosophical to blow with a pair of com- 

 mon bellows upon the rays of the sun, under the expectation 

 of putting out the heat. But, be this as it may, the effect is most 

 decisive ; for, as it will presently appear, this useful household 

 instrument, like the traveller in the fable, blo'ws both hot a7id 

 cold. 



After the idea of using the bellows had suggested itself to 

 my mind, I took the first opportunity which occurred of 

 placing a delicate thermometer in the focus of the condensing 

 lens of my solar microscope. By directing a current of air 

 upon it, the calorific energy of the solar beam was suddenly 

 diminished, and, after a few blasts, the thermometer indicated 

 about 60° of temperature only. It is quite evident that no- 



Third Series. Vol. 10. No. 60. March 1837. 2 B 



