ON THE CHOICE OF A MICROSCOPE. 161 
should be of flint glass; and secondly, for his happy concep- 
tion of uniting these lenses, already identical as at c, still 
more perfectly by a transparent and colourless cement; thus 
greatly diminishing the loss of light by reflection, and which 
is considerable at the numerous surfaces of an achromatic 
combination. Mr. Ross’s, for being the first to notice that 
however perfect the correction of an object-glass might be, 
*‘the circumstance of covering the object,” to use his own 
words, “‘ with a piece of thinnest glass or talc, disturbed the 
corrections if they had been adapted to an uncovered object, 
and rendered an object-glass which was perfect under one 
condition seriously defective under the other.’ It was to 
correct this defect that he devised the well-known contrivance 
called the adjustment of object-glasses, by which they are 
rendered equally correct whether for covered or uncovered 
objects; and for the particulars of which, and further im- 
provements in its mechanism by Messrs. Powell and Smith, 
the reader is referred to Mr. Quekett on the ‘ Microscope,’ 
and to Mr. Andrew Ross’s invaluable paper in the ‘ Penny 
Cyclopedia,’ the substance of which is embodied in both 
Dr. Carpenter’s and Mr. Quekett’s works. 
I have deemed it proper to allude thus far to the history 
and make of the achromatic object-glass,* not only because 
* The following diagrams of the different object-glasses will help to explain 
the complex structure and consequent costliness of an achromatic combina- 
tion to those who are unacquainted with it. 
2 . < i. * 2 
3 in., 2 in., 14 in. lin, 3. 4, sh 2 3,2 3) op oe Fee 
All the double convex and plano-convex lenses are of crown glass, the 
Jano and double concave and meniscus lenses being of flint glass. It will 
a seen, therefore, that each of the object-glasses, from the 3 to the 4th, are 
made up of as many as eight distinct lenses: the back combination being a 
triplet composed of two double convex lenses of crown, with a double 
concave lens of flint glass between them; the intermediate combination 
