24 PEOFESSOE TYNDALL ON CALOEESCENCE. 



Perfect imperviousness might doubtless be secured by augmenting the quantity of green 

 colouring-matter. It is with glass of this description that the carbon should be mixed 

 in the construction of black-bulb thermometers ; on entering such glass the solar rays 

 would be entirely absorbed, and greater diiFerences than those now observed would 

 probably be found to exist between the black-bulb and the ordinary thermometer. 



In conclusion, it gives me pleasure to mention the intelligence and skill displayed by 

 my assistant, Mr. Barrett, in executing the numerous experiments committed to his care 

 during the progress of this investigation. 



