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OPTICS 



THE science of the nature and laws of vision, a 

 mixed mathematical science, which includes 

 catoptrics, dioptrics and perspective, and con- 

 siders the nature, composition, and motion of 

 light. 



Seeing a Star when not looking at it. 



By directing the eye to another part of the field of 

 a telescope, a faint star, in the neighbourhood of a 

 large one, will often become very conspicuous, and 

 yet totally disappear if the eye be turned full upon it. 



The Retina. 



An impression on the retina continues for about 

 the eighth part of a second after the object that pro- 

 duced it is withdrawn. Thus the luminous ring 

 formed by whirling a burning stick. 



Intense Light. 



After the trial of various pyrotechnical prepara- 

 tions, Lord Drummond was led to adopt a ball of 

 lime intensely ignited, and placed in the focus of a 

 parabolic mirror. The intensity of the light from the 

 ball is about 75 times greater than that of an Argand 

 lamp. It was used on the survey of the coast of 

 Ireland. 



