PREFACE. 



Y N the preface to my Essays on Electricity and 

 JL Magnetism, I informed the public that it was my 

 intention to publifti, from time to time, eflays defcribing the con- 

 ftru&ion and explaining the ufe of mathematical and philofophi- 

 cal inftruments, in their prefent ftate of improvement. This 

 work will, I hope, be confidered as a performance of my promife, 

 fo far as relates to the fubjecl: here treated of. 



The Brit chapter contains a fhort hifiory of the invention and 

 improvements that have been made on the microfcope, and 

 Father Torre's method of making his celebrated glafs globules. 

 The fecond treats of vifion, in which I have endeavoured to ex- 

 plain, in a familiar manner, the reafon of thofe advantages which 

 are obtained by the ufe of magnifying lenfes ; but as the reader is 

 fuppofed to be unacquainted with the elements of this fcience, fo 

 many intermediate ideas have been neceflarily omitted, as mult 

 mfome degree leffen the force, and weaken the perception of the 

 truths intended to be inculcated: to have given thefe would have 

 required a treatife on optics. 



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