CONCERNING SCIENTIFIC APPARATUS. 521 



4. Phosphorescent, fluorescent, and calescent bodies. 



5. Opaque screens, diaphragms, and slits. 



6. Regulators. The iris of the eye. 



7. Photometers, photographic apparatus, actinometer, thermopile, Bunsen 



and Roscoe's photometer, selenium photometer, Crookes' radiometer, 

 and other instruments. 



8. Fraunhofer's lines of reference, and maps of the spectrum. Standard 



sperm candle, burning 120 grains of sperm per hour. 



SIGHT. 

 PHYSIOLOGICAL ASPECT OF OPTICS. 



Apparatus for determining the conditions of 



1. The visibility of objects, with respect to size, illumination, &c. 



2. The perception of the distinction of objects. 



3. The perception of colour, as depending on the composition of the 



light coming from the object. 



Apparatus for comparing the intensity of luminous impressions, as depending 

 on the intensity of the exciting cause and on the time during which it has 

 acted, and for tracing the course of the development of a luminous impression 

 from its first excitation to its final decay and extinction. 



Ophthalmometers, for measuring the dimensions of the eye and determining 

 its motions ; and for ascertaining the two limits of distinct vision. 



Ophthalmoscopes, for illuminating and observing the interior of the eye. 



BIOLOGICAL APPARATUS. 



1. For measuring the ingesta, egesta, and weight of living beings. 



2. For testing the strength of animals and measuring the work done 



by them. 



3. For measuring the heat which they generate. 



4. For determining the conditions of fatigue of muscles. 



5. For investigating the phenomena of the propagation of impulses through 



the nerves, and of the excitement of muscular action. 

 VOL. II. 66 



