ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
409 
attribute this partly to the fact that in English objectives the spherical 
and/or chromatic aberration is often not entirely corrected, some being 
intentionally left for correction by a contrary error in the eye-piece ; 
but chiefly to the object-glass being adjusted to project the image a fixed 
distance, which is generally 10 in., that being the usual length of the 
English Microscope-tube : but when the image is projected not to 10 in., 
but to a distance considerably exceeding this, say to a .distance of 40 or 
50 in., the corrections are altogether disturbed, and the delineation in 
consequence deteriorated. A main cause of the disturbance can be 
removed in object-glasses provided with a collar adjustment for cover- 
correction, by altering the relative distance of the different combinations 
of lenses in the object-glass ; and I have had even a 1J in. objective 
mounted so that the distances between the lenses could be changed ; 
but other causes of disturbance are left, or even increased, by the 
change, and the image is never so clear as it is at the 10 in. A pro- 
jection eye-piece, however, avoids this difficulty, for it takes up the 
image at the proper distance, and is furnished with means for adjusting 
its own action to whatever distance the sensitive plate may be placed. 
I have used Zeiss’s, but I believe several English makers supply similar 
ones. You will see that there are two combinations of lenses, the 
distance between which can be regulated, and the adjustment thereby 
effected. It is correct when the edge of the field is sharp and clear. 
The method of illumination may vary according to the work to be done. 
For moderate magnification, say up to about 300 diameters, I have found 
diffused daylight from cloud or blue sky to give good results. The same 
light, or a good lamp, can also be used for higher magnification, 500 or 
even 1000 diameters ; but the light is then so feeble that focusing is 
difficult, and a very long exposure necessary. I show you one photo- 
graph of a Triceratium, x 1000, taken with diffused daylight, for which 
the exposure was 1 hour 40 minutes ; and another of an Arachnodiscus, 
X 800, taken with a paraffin lamp, and an exposure of 1 hour 20 minutes. 
With such prolonged exposures the chances of vibration, or of changes 
of focus arising from the expansion or contraction of the instrument in 
consequence of variations of temperature, are greatly increased ; so that 
the final result is seldom so clear and sharp as with a more intense illu- 
mination and shorter exposure. For high magnification, therefore, oxv- 
hydrogen light is usually employed ; and better even than this I consider 
sunlight. It has, of course, some serious disadvantages. It cannot be 
obtained whenever you happen to require it by merely turning on a tap. 
You are dependent upon the clerk of the weather ; and when you do get 
it, it is too apt to be intermittent. Many a time I had to wait patiently, 
waiting for a break in the clouds. But when you do get it, I think it is 
the ne plus ultra. When using it, exposures can be reckoned by seconds, 
and I have a negative of Pleurosigma angulatum, good so far as density 
is concerned, taken with an exposure of only one second to sunlight, on 
an ordinary Ilford plate. Whether the source of illumination be a lamp, 
or a lime cylinder, or the sun, care must be taken so to focus the sub- 
stage achromatic condenser, that an image of the source of illumination 
is thrown on the exact plane of the object. This is all-important. 
I will now try to describe, with some minuteness, my course of pro- 
cedure when taking a photograph by sunlight, premising that my objects 
