PROCEEDINGS OF THE SOCIETY, 715 



Mr. Crisp described Professor Eanvier's method of fine adjust- 

 ment by the eye-piece. 



Mr. Ingpen thought the use of the eye-piece for the purpose 

 suggested would have a deteriorating effect on the definition, particu- 

 larly with high powers of large aperture, which were adjusted to a 

 certain length of body. 



Dr. Maddox, in exhibiting two photomicrographs of Pleurosigma 

 angulatum, magnified 3000 diameters, taken above nine or ten years 

 since with a Wale's |- photographic objective and amplifier, said, " I 

 do so, believing they may be of interest, as exhibiting markings 

 between the dots, which I venture to suggest may correspond to the 

 small markings between the hexagons, to which Mr. Stephenson called 

 attention in his paper of June 5th, 1878, entitled ' Note on the Effect 

 of Pleurosigma angulatum and other Test Objects by excluding the 

 central Dioptric Beam of Light,' and to the figure given by Dr. Alfred 

 Eichorn's mathematically calculated drawing, produced by six equi- 

 distant spectra in a circle. 



" Mr. Stephenson in his paper remarks that * such markings would 

 possibly have escaped observation altogether had not mathematical 

 theory pointed out that such images ought to appear.' 



" Mr. Stephenson showed these markings by a large-angled oil- 

 immersion lens, the centre stopped out at the back of the objective 

 by a stop of black paper with marginal openings ; and in the April 

 number of the Journal of the Society, at page 359, in the article 

 entitled ' Notes on Aperture, &c.,' by the editor, Mr. Crisp, the figure 

 of Dr. Eichorn's drawing is repeated with the following remarks, ' The 

 special feature was the small markings between the hexagons which 

 had never been seen on Angulatum,^ with the statement that ' the small 

 markings were found actually to exist, though they were so faint that 

 they had escaped observation until the result of the mathematical 

 deduction had shown that they ought to be seen.' 



" As these photomicrographs were produced so long back as to be 

 some time in advance of the papers alluded to, and if, as I venture to 

 suppose, the markings represented be the identical markings so 

 happily shown by Mr. Stephenson, they tend to prove the value of 

 photomicrography. 



" The difficulty attendant on producing in the negative the six 

 small interspaced markings was extreme. I took twenty-one, if not 

 twenty-three, negatives of the same frustule mounted dry on the cover 

 before obtaining the markings in the print as bright spots, and then 

 only on small parts of the frustule. 



" I have had the pleasure of obtaining Mr. Stephenson's opinion 

 after seeing the photomicrographs. He thinks the markings larger 

 than what he exhibited in the Microscope, and suggests this may be 

 caused by being less sharp. He also doubts if they be the same, as 

 they were produced by totally different conditions, and suggests that 

 I may have used seven diffraction pencils. To me they appear to 

 occupy the position given to them in the calculated figure. I regret 

 the photographs are so considerably faded. They were unfortunately 



