20 SCIENTIFIC APPARATUS. 



6. Regulators. The iris of the eye. 



7. Photometers, photographic apparatus, actinometer, 

 thermopile, Bunsen and Roscoe's photometer, sele- 

 nium 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 



i. The visibility of objects, with respect to size, illumina- 

 tion, &c. 



2. The perception of the distinction of objects. 

 3. The perception of colour, as depending on the composi- 

 tion 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. 



i. 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. 



