VIU PREFACE. 



the images, formed by lenses and mirrors, has been correGtly investi- 

 gated, and thcs inaccuracy of some former Estimations has been de- 

 monstrated. 



In the department of physical optics, the phenomena of halos and 

 parhelia have been explained, upon principles not entirely new, but 

 long forgotten : the functions of the eye have been minutely examined, 

 and the mode of its accommodation to the perception of objects at 

 different distances ascertained : the various phenomena of coloured 

 light have been copiously described, and accurately represented by 

 coloured plates; and some new cases of the production of colours 

 have been pointed out, and have been referred to the general law of 

 double lights, by which a great variety of the experiments of former 

 opticians have also been explained ; and this law has been applied to 

 the establishment of a theory of the nature of light, which satisfacto- 

 rily removes almost every difficulty that has hitherto attended the 

 subject. 



I'he theory of the tides has been reduced into an extremely simple 

 form, which appears to agree better with, all the phenomena, than the 

 more intricate calculations which they have commonly been supposed 

 to require. With respect to the cohesion and capillary action of 

 liquids, I have had the good fortune to anticipate Mr. Laplace in 

 his late researches, and I have endeavoured to show, that my assump- 

 tions are more universally applicable to the facts, than those which 

 that justly celebrated mathematician has employed. I have also at- 

 tempted to throw some new light on the general properties of matter 

 in other forms : and on the doctrine of heat, which is materially con- 

 cerned in them ; and to deduce some useful conclusions from a com- 

 parison of various experiments on the elasticity of steam, on evapora- 

 tion, and on the indications of hygrometers. I have enumerated, in 

 a compendious and systematical form, the principal facts which have 



