ficT. v.] DISSERTATION SECOND. 193 



After the publication of the Dioptricks of Descartes, in 

 1637", a considerable interval took place, during which, op- 

 ticks, and indeed science in general, made but little pro- 

 gress, till the Optica Promola of James Gregory, in 1063, 

 seemed to put them again in motion. The author of this 

 work, a profound and inventive geometer, had applied 

 diligently to the study of opticks and the improvement of 

 optical instruments. The Optica Promota embraced seve- 

 ral new inquiries concerning the illumination and distinct- 

 ness of the images formed in the foci of lenses, and con- 

 tained an account of the Reflecting Telescope still known 

 by the name of its author. The consideration which sug- 

 gested this instrument was the imperfection of the images 

 formed by spherical lenses, in consequence of which, they 

 are not in plane, but in curved surfaces. The desire of 

 removing this imperfection led Gregory to substitute re- 

 flection for refraction in the construction of telescopes ; 

 and by this means, while he was seeking to remedy a 

 imall evil, he provided the means of avoiding a much great- 

 er one, with which he was not yet acquainted, viz. that 

 which arises from the unequal refrangibility of light. The 

 attention of Newton was about the same time drawn to the 

 same object, but with a perfect knowledge of the defect 

 which he wanted to remove. Gregory thought it necessa- 

 ry that the specula should be of a parabolick figure ; and 

 the execution proved so difficult, that the instrument, dur- 

 ing his own life, was never brought to any perfection. 

 The specula were afterwards constructed of the ordinary 

 spherical form, and the Gregorian telescope, till the time 

 of Dr. Herschell, was more in use than the Newtonian. 



Gregory was professor of mathematicks at St. Andrews, 

 and afterwards for a short time, at Edinburgh. His writings 

 strongly mark the imperfect intercourse which subsisted 

 at that time between this country and the Continent. — 



