yom, 
On the Achromatic Microscope. 265 
= ‘ 
san vk —On ae Achromatic Microscope ; i the Tourist of Sei- 
ence, by Epwarp date 
Is a former stile on improvements i in sltirscope, published 
in Vol. XTX. of the Journal of Science, a description was given of 
the best form of the achromatic microseaiie then adopted. Material 
improvements have since been made, which it is the object of this 
paper to explain.’ Many of these have been introduced by Alden 
len. ; ms ’ ; 
When a lens is required with a focal distance of an inch or more, 
and is to be used as a simple microscope, one single lens of plate 
glass, having its radii nearly as 1 to 6, will exhibit objects about as 
distinctly as one of any other construction. But when the focal dis- 
tance is half an inch or less, an achromatic Jens is superior, whether 
for simple or compound microscopes. ‘The form represented in 
Fig. 1, is. the best for simple microscopes, when the focal distance is 
from one half to one fourth of an inch. If a shorter focal distance 
be required, and a large aperture is necessary, it will be “a advan- 
lageous to employ the sextuple achromatic shown in Fig. 
In Fig. 1, is shown the best form of the object glass of. a GoD 
microscope, when an aperture is required nearly equal to one half of 
the focal distance : a is the first surface, 6 the last, and o the place 
Of the emgect to be viewed. The following are the dimensions and 
radii, id 
as 
Inch. | Fig. 43> 
Radius of the Ist surface, . 0.15 
do, 2d do. = * sion 
do. 3d_—s do. 152 
do. 4th do. 0. 15 § flint. 
do. Sth. «do. =~. 
do. Gh ‘do. ogngee 
Focus of the compound lens, 0.27 
Diameter, .: . | ap... 0.14 amy ; 
Clear aperture, . =} 10 to 0.12 " 
ls ee 
* The figures are all drawn four times the real size 
