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effect at the back of the objective, such that the dioptric beam 

 was at the extreme edge of the lens and the first diffraction 

 spectrum at the opposite edge of it, then a glance at the aperture 

 table in the " R. M. S. Journal " would show you that the 

 striation was a little less than 48,000 per inch. But if, when a 

 narrow angled central beam were used, both the 1st order 

 spectra were at the opposite edges of the lens, then the striation 

 would be somewhat less than 24,000 per inch, viz., half the 

 amount given in the table. If only, therefore, the N.A. of the 

 objective is known a rough estimation of the fineness of the 

 striation may be arrived at by inspection, without the necessity 

 of employing micrometers. The error of the estimation will 

 be within the variation of the fineness of the striation of the 

 diatomacese. Of course it is better to select such an objective 

 as will show the spectra at the edge of its aperture with central 

 light, or the dioptric beam and a spectrum with oblique light. 

 Mr. Ingpen's medium was an exceedingly valuable one, but he 

 was very much afraid that it was only good for temporary 

 purposes. All his own slides had gone bad except one mounted 

 by Prof. Hamilton Smith and one mounted by Father Thomp- 

 son. Of those sent over by Prof. Hamilton Smith only 

 four or five out of a box full remained uncrystallized. Of 

 these one was still intact, but the rest showed signs of crystal- 

 lization. It was a great pity that so useful a medium should 

 be so uncertain, but he feared that all these arsenical prepara- 

 tions were wanting in permanence. 



Mr. Ingpen said that compounds of sulphur and arsenic 

 alone were more permanent than those containing bromide. 

 He could quite endorse the President's remarks as to quinidine, 

 to which his attention was called on finding that piperine was 

 so irrational that it was of little use for photography. One of 

 the elements of success or failure was to be found in what the 

 diatoms had been kept in previous to mounting. A lot mounted 

 by Mr. Suffolk, which had never been put into anything but 

 weak alcohol, stood very well, whilst some more which had 

 been cleaned and acidulated would not stand. Of his specimens 

 in quinidine mounted years ago some were still good, and others 

 completely gone. A second or third melting seemed essential 

 to success ; a single melting was not sufficient to preserve them. 



Mr. Karop said he had some experience with quinidine. The 



