T. F-. SMITH ON THE USE OF ISOCHROMATIC I'LATES. 185 



matic plate only. It is not for me to say how tliis is brought 

 about, but content mj^self with announcing it as a fact, and 

 leave it to those with more special knowledge to explain why. 



Prints Nos. 2 and 2a show the Podura scale, taken at about 

 1,100 diameters, with a -g^in., and shows the same difference of 

 result when taken on the two sorts of plates. 



Nos. 3 and Sa are prints of Goscinodiscus asteromphalns, taken 

 at 1,750 diameters with a cheap xVin. oil immersion, and here 

 the difference in focus is simply that between a positive and a 

 negative image of the same diatom when taken first on an 

 isochromatic, and secondly on an ordinary plate, the former 

 being the image j^roduced on the screen of the camera. 



All these were taken without any screen whatever, but I have 

 other prints here, taken of histological subjects with the same 

 lenses in which the yellow screen has been used, not, however, 

 to produce a sharper focus, but to render certain colours in the 

 objects more or less actinic. Here a screen certainly is neces- 

 sary, but, as I have often found when leaving it out accidentally, 

 it makes no difference in the focus whatever. 



I can qiiite bear out Mr. Gill's experience that on suitable 

 subjects almost, if not quite, as good results can be obtained 

 with ordinary objectives as with the more expensive apochro- 

 matics, and I may add to this that I have found it a great con- 

 venience to be able to include more of the object by not using 

 the eye-piece, but this, with me, only applies to the lower 

 powers, as I find that when using a wide angled ^in. ory^in. this 

 way the curvature of the field more than neutralizes the 

 increased image taken in. 



In conclusion, I may say that I bring this subject before j^ou 

 in no controversial spirit, but only to elicit the truth, whatever 

 it may be. 



