[ 335 ] 

 LV. An Account of Test Objects for Microscopes. By Andrew 



PltlTCHARD, Esq.*. 



[With a Plate.] 



TpVERY important advance in our knowledge of those 

 l *- J bodies in the material universe, from which our earth ap- 

 pears as an atom, has been coeval with, and greatly dependent 

 upon, some augmentation of the powers and effectiveness of 

 telescopes. Before the discovery of the double stars and ne- 

 bulae, the goodness of these instruments was determined by 

 their capability of showing the planets and their satellites. 

 But, since our acquaintance with the former bodies, telescopes 

 have to undergo more severe tests, and greater accuracy in 

 their construction is required. What has been advanced in 

 regard to the telescope will be found applicable to the mi- 

 croscope ; and to the discovery of certain objects which may 

 be considered as tests of the penetrating and defining powers 

 of this instrument, we may justly attribute the grand and mag- 

 nificent improvements which the microscope has recently re- 

 ceived. 



In the perusal of the works of Leeuwenhoek, Dr. Goring 

 met with a passage describing the dust, or imbricated scales, 

 which cover the wings of the silkworm (Phalena Mori), from 

 which he was led to suspect there were some peculiar pro- 

 perties in the lines on the feathers and scales of insects, ren- 

 dering them more difficult to be discerned than other micro- 

 scopic objects ; and the result of his investigation was the dis- 

 covery of their properties as tests — a description of object be- 

 fore unknown in the annals of microscopic science. 



Now, as it is undoubtedly of the highest importance to the 

 naturalist that he should know the exact capabilities of his 

 instrument, in order that he may not be led astray in his in- 

 vestigations, by placing undue confidence in it; and as these 

 tests offer the best means of accomplishing this end, I conceive 

 them to be of the greatest value and interest. As no complete 

 account of them is extant, I shall endeavour to supply this de- 

 ficiency in the present chapter, and illustrate the subject by 

 accurate drawings of them, greatly magnified. * * * 



Having ascertained that different test-objects require dif- 

 ferent degrees of perfection in the instrument used to develope 

 their structure, it became an interesting pursuit to discover 

 those which are best adapted for this purpose, and the pecu- 

 liarities, in the illumination, &c. under which they are exhi- 



* Abridged from the "Microscopic Cabinet." London: Whittaker, Treacher 

 and Co. 1832. An account of this work was given in the Lond. and Edinb. 

 Phil. Mag. and Journ. vol. i. p. 163. 



