DISCOVERT BY MEANS OF NEW OPTICAL INSTRUMENTS. 477 



more powerful telescope, it was seen magnified, and two 

 days afterwards he perceived that it had changed its 

 place.' Thus was Uranus discovered as a planet. ' The 

 calculations of the perturbation of the planet enabled 

 astronomers to discover that it had been observed as a 

 star in three different positions in former times, namely, 

 by Flamsteed in 1690, by Mayer in 1756, and by Le 

 Monnier in 1769.' l 



Who really invented the microscope, is not known 

 with certainty, but Malpighi (in 1661) was one of the 

 first to make discoveries by its assistance, and the first to 

 employ it in examining the anatomy of insects. Grew 

 also (about 1670) discovered the stomates in the leaves of 

 plants, and both he and Malpighi discovered much re- 

 specting the structure of the cellular tissue of plants, by 

 the aid of that instrument. By similar means, Malpighi 

 discovered many new facts respecting the gradual growth 

 of seeds; and Leeuwenhoeck (about 1680) discovered 

 animalculse, and that a particle, no larger than a grain of 

 sand, of the roe of a cod-fish, contained about 10,000 ova. 



6 John Dollond, in 1757, found that when an object 

 was seen through two prisms, one of glass and one of 

 water, of such angles that it did not appear displaced by 

 refraction, it was colourless. Hence it followed that with- 

 out being coloured the rays might be made to undergo 

 refraction ; and that thus, substituting lenses for prisms, 

 a combination might be formed which should produce an 

 image without colouring it, and make the construction of 

 an achromatic telescope possible.' 2 Hall, in 1733, had 

 also constructed achromatic lenses, but did not publish his 

 discovery. For remarks respecting various improvements 

 which have been made in astronomical apparatus and 



1 Whewell, History of the Inductive Sciences, vol. ii. 3rd ed. p. 177. 



2 Ibid. p. 289. 



