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 now obtained with us by some of our leading opticians, 
and by Nachet in France, and Spencer in America. 
To obtain object-glasses of good defining 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 
be done, he should provide himself 
with some means of artificial illumi¬ 
nation. A wax candle 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 sabject should carefully 
eruse the able memoir of the President of the Royal Irish Academy, read before that 
ody, 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. 
Fisr. 5. 
