UUIVEHSITY 



OPTICAL INSTRUMENTS. 



INTRODUCTION. 



THE construction of optical instru- 

 ments has, in almost every instance, 

 originated with eminent philosophers 

 and mathematicians. Their gradual 

 perfection has been a natural result of 

 the difficulties which were presented to 

 the progress of discovery, by the in- 

 efficient and inaccurate means which 

 science possessed ; and thus, the same 

 great minds that have struck out and 

 pursued vast and splendid ideas in their 

 investigations of nature, have only been 

 enabled to follow up their own concep- 

 tions by applying themselves to the 

 practical improvement of the instru- 

 ments with which they had commenced 

 their discoveries. For instance, we are 

 indebted to Newton for the construction 

 of the first reflecting telescope that was 

 ever made, although the idea had been 

 previously suggested by Dr. Gregory. 

 Leuwenhoek, one of the most assiduous 

 naturalists of his day, earned on his 

 curious researches in the animal and 

 vegetable economy, with microscopes 

 made by his own hands. The late Dr. 

 Herschel, whose astronomical disco- 

 veries were the result of the profoundest 

 mathematical knowledge, constructed 

 the most powerful telescopes ever 

 known, which, like the Gregorian, bear 

 the name of their inventor. Indeed, the 

 ordinary makers of optical instruments 

 have been often men of considerable sci- 

 entific attainments ; and it is from this 

 union of a theoretic and practical know- 

 ledge, that these instruments, as is the 

 case with almost every other important 

 invention and improvement, have been 

 conducted to their present very high 

 state of perfection, in an almost un- 

 limited adaptation to all the purposes of 

 science, and all the wants and luxuries 

 of common life. 



In the limited extent of this treatise 

 on Optical Instruments, the chief object 

 will be to point out the principle of 

 their construction, freed from technical 



mathematics, and to describe the im- 

 proved state in which the more im- 

 portant are adapted to their several 

 purposes. The various modes in which 

 many instruments, not differing in their 

 principle and end, are mounted, depend 

 upon the varying taste of the artist, the 

 caprice of fashion, or the demands of 

 luxury. Any minute details of the ex- 

 ternal parts of the instruments would 

 be therefore perfectly useless. Indeed, 

 the external parts of the ordinary in- 

 struments can be much better under- 

 stood by an inspection at the opticians* 

 shops, than in volumes of description. 

 So indifferent is the outward form of 

 the common instruments, that makers 

 living even in the same neighbourhood 

 vary as much in their mountings as the 

 artists of London or Paris. We shall, 

 therefore, confine our observations to 

 the essential parts in the construction, 

 of particular instruments, passing over 

 their accidental varieties of mounting ; 

 in the same way that the anato- 

 mist speaks of the various parts of the 

 human body, without enquiring whether 

 the subject upon which he has made his 

 observations was dark or fair, tall or 

 short. 



The ancients seem to have been but 

 little acquainted with dioptrical instru- 

 ments, or those by which the light is 

 refracted and transmitted; from their 

 earliest history, however, they appear 

 to have been conversant with the laws 

 of the reflection of objects from the sur- 

 face of water and polished metals, or 

 that department of optical science called 

 catoptrics. The first application of their 

 knowledge of this branch of science, with 

 which we are acquainted, is that of the 

 burning mirrors employed by Archi- 

 medes, a philosopher of Syracuse, about 

 200 years before the Christian era, who, 

 at the siege of that city, by Marcellus, 

 the Roman Consul, employed them to 

 destroy the besieging navy. The me- 

 thod by which this was probably accom- 

 plished is thus described by the histo- 



