REVIEWS. 33 



used as tests, are rendered visible. Complete freedom from aberration 

 and high magnifying power are not sufficient alone to give perfect vision to 

 this class of objects ; these essential qualities must be combined with a 

 large angle of aperture,* the attainment of which is of the utmost import- 

 ance in the construction of object-glasses. Within the last few years asto- 

 nishing perfection has been attained by some of our best makers. Micro- 

 scopists are still alive who can remember the admiration excited by the 

 first objectives of 20 or 30 aperture; and when Ross carried this ele- 

 ment to 60, it was considered a ne plus ultra. Angles of 170 and 

 upwards have been HOAV obtained with us by some of our leading opticians, 

 and by Nachet in France, and Spencer in America. 



To obtain object-glasses of good denning power should be the first aim 

 of the student ; the form of stand, provided it is free from tremor, &c., is 

 only a secondary consideration in comparison. The powers most useful are, 

 the inch, half-inch, and quarter-inch. He will rarely need higher powers. 



Whenever practicable the student should carry on his researches by day- 

 light ; but, as in all cases this cannot Fijr ft 

 be done, he should provide himself 

 with some means of artificial illumi- 

 nation. A wax caudle gives a good 

 light ; but a much more agreeable light 

 is furnished by a lamp that known 

 as the Cambridge is the best form if 

 oil is used. In towns where gas can 

 be obtained, an achromatic lamp, 

 adapted to its use by Mr. Highley, gives 

 the most perfect means of artificial illu- 

 mination we are acquainted with. It 

 consists of a stage, supported by a tube 

 and socket (see fig. 5), and carrying 

 an Argand burner. A metal cone rises 

 to the level of the burner, and is about 

 an eighth of an inch from its outer 

 margin, by which means a bright cylin- 

 drical flame is procured. A Leblond's 

 blue glass chimney is placed over the 

 burner, which corrects the colour of the flame, and this is further rectified 



* Those who feel an interest in the investigation of this subject should carefully 

 peruse the able memoir of the President of the Royal Irish Academy, read before that 

 body, January 23, 1853, and printed in its " Proceedings," vol. vi., part 1, pp. 38-47, 

 where the causes of imperfection in some glasses of high angular aperture is investigated. 



