1898,] MICROSCOPICAL JOURNAL 21 



sharply defined on the ground-glass. I found that this 

 was at a distance of 8 in. from the lens, and concluded 

 that this is the distance of the virtual image which I see 

 when I use the microscope. I then substituted a fresh 

 object on the stage of the microscope, carefully focussed 

 it, replaced the camera without focussing it, inserted a 

 sensitive plate, exposed, and developed it, getting a very 

 much better image than hitherto, really sharp, with a 

 power of 55 diameters. I next attempted a very difficult 

 object, Pleurosigma angulatum, the objective being a 

 Beck ^in., and the eyepiece a fin. Huygheniau belong- 

 ing 10 the telescope. This was carefully focussed, and 

 the camera reapplied, with the result that the negative 

 shows the diatom dotted all over the field. Those who 

 know the difficulty of seeing the dots in this object will 

 understand the severity of this test. The lens of the 

 camera was a plano-convex achromatic objective from an 

 opera-glass, 4^in. focus, the distance of the objective 

 from the plate being about lOJin. Shortly afterwards I 

 substituted a stage micrometer for the diatom slide, and 

 found that the actual magnifying power was 1,000 dia- 

 meters. 



Methods in Microscopical Technique. 



I. Down to recent times the microscope was utilized 

 by means of various optical accessories and their devel- 

 opment was a matter of physics. Methods of manipu- 

 lation then in vogue have since largely fallen into disuse 

 and their votaries have become less in number. Mr, H. 

 B. Ward cites as an evidence of the failure of that line 

 of development the fact that diatomists cannot agree re- 

 garding the interpretation to be put upon a direct image 

 presented by the microscope. The followers of this 

 school took their objects for stady as they found them in 

 nature and without preparation of any sort. 



