444 PROCEEDINGS OF THE SOCIETY. 



Dr. Scales said he did not consider that it was quite fair to Mr. Hart- 

 ridge to confront him with Mr. Gordon'^ criticisms as to the difficulties 

 offered in his paper without first having let him see the letter in which 

 they were contained. He had, however, only received the letter the 

 previous day, so that it would hardly have helped matters to have for- 

 warded it at so late an hour. In regard to some of these criticisms, he 

 realized that Mr. Hartridge had a large amount of work yet to perform 

 in connexion with his method, even after the primary difficulties had 

 been overcome, and pressure of work prevented Mr. Hartridge from 

 working at these difficulties at the moment. He had, therefore, him- 

 self suggested the present preliminary communication, and he thought 

 that the ingenuity of the method and apparatus would arouse much 

 interest. He did not himself propose to offer any criticisms. 



Mr. Beck, while admiring the ingenious way in which the optical and 

 mechanical details of Mr. Hartridge's apparatus had been carried out, 

 wished to point out that the various component lenses of tlie collimator 

 and telescope had a double purpose to perform, this being due to the 

 fact that the instrument was a Microscope and a spectrometer at one 

 and the same time. Now, the correction of a lens in regard to one 

 focal plane is frequently carried out with a corresponding sacrifice in its 

 definition in another. He would therefore hke to ask Mr. Hartridge 

 whether this difficulty had been met with, and how far aberrations thus 

 introduced had affected the interpretation of results, and how they had 

 been overcome in constructing the present apparatus. 



Mr. Hartridge said, in reply, that in practice no difficulty had been 

 met with from this cause. It was necessary to give certain lenses a 

 double duty in this way in order to reduce the loss of light at glass-air 

 surfaces to a minimum. In fact, by far the most important improve- 

 ments that had been made in the apparatus had been the simplification 

 of the lens system by which the various images were produced. The 

 reason why no special difficulty had been experienced in practice in olj- 

 taining good images of the spectra could probably be accounted for as 

 follows : — 



1. Although the correction of chromatic aberration is carried out 

 in any lens used in the optical system, yet it is further rendered of no 

 account because of the exclusive use of monochromatic light, when 

 spectral measurements were being made. 



2. In both the high-power lenses, viz. the Conrady condenser and 

 the objective, the correction of zonal spherical aberration is carried out 

 to the highest degree ; this coupled with the fact that the lenses also 

 comply with the sine condition and are focused on the same point, 

 necessitates that an image received in the lower focal plane of the con- 

 denser shall be transmitted as a true image to the upper focal plane of 

 the objective. 



3. The only other lenses used in forming the images of the spectrum 

 are those lenses of low power, and these lie outside of the optical path 

 of the rays which form the magnified image of the diatom. They may 

 therefore have surfaces, the curves of which are best suited to the forma- 

 tion of sharp spectra. 



