CORRESPONDENCE. 25 



I pass tbe light tlirougli a paraboloid, of Mr. Nacliet's niako, 

 of Paris : this glass is similar to the paraboloid of the principal 

 English opticians, but somewhat smaller in size. 



When well centred, and in focus, the paraboloid gives a very- 

 white light with high power ; and every object that comes under the 

 eye is wonderfully clearly and perfectly illuminated ; but if the object- 

 glass is not well corrected for the thickness of the glass cover it will 

 at once be apparent, because then you will have no perfect image at 

 all, but an unseemly object, the markings of which will become the 

 more apparent the more you approach the true correction to be made, 

 and then you will see every particle of the object strikingly well 

 defined, and every scale of many, if not of all insects, will appear 

 covered with dots or beads. 



I think it was due to an imperfect correction that Mr. Mclntire 

 was not always able to see the beaded appearance of the scales. 



Here follows a list of butterflies or moths whose scales I have 

 particularly observed, and which wear most clearly the beaded appear- 

 ance, with nothing nwre to do to show them but a just correction of 

 the object-glass : — Grapliohjtha minutana, Pterophorus pterodactyl us, 

 Platyptilus pilosellce, Alucita hexadactylus, ClecopJiora luduosella, Har- 

 pella hractiella, Tortrix Hoffmanseggana, Vanesse Atalanta, Moro-sphinx, 

 Cramhus, Minophora metaxella, Vanesse gamma, Cosmophora composana, 

 Serlcosis urticana, Liparis chrysonhaca, Podalirius, Tponomerita mali- 

 nella, Vanesse urticce, Yponomerita padilla, Orthosia Gothica, Coccyn 

 Buolcann, C^ilyade-Hyale, Apatura Iris, Agravlis vanillce, Pheteimr, 

 Deilephila Elpenor, Melanippe, Atlas, MorpJio Helinor, Junonia Geno- 

 veva, Lycorea atorgatis, Alcandor, Erytlionius, and many more, which 

 are either unknown to me, or whose names are of no scientific nomen- 

 clature to be here inserted. 



In confirmation of the present letter, I beg the Eoyal Society to 

 accept a dozen of my slides of butterflies' scales, some of them selected. 

 They are of the Melanippe (Brazil), Alcandor (China), Deilephila 

 Elpenor, Tthetenor, Papilio agaris, Junonia Genoveva, Lycorea ater- 

 gatis. Atlas. I hope they will be graciously accepted. 



Using the same mode of illumination with the paraboloid, I have 

 tried to discover the real natiu-e of the markings of the Snrirella 

 gemma, that most beautiful puzzle, and I have clearly seen the dots 

 which this interesting diatom wears ; they are similar to those of the 

 Pleurosigma Balticum, and similarly disposed ; that is to say, they 

 make lines which intersect each other at right angles. 



This appearance of the Surirella gemma must be the true one, 

 because, having tried that same diatom with a double reflector obliquely 

 disposed at right angles to each other, I have obtained the same 

 appearance. 



So I cannot sufficiently recommend the use of the paraboloid ; it 

 gives a beautiful dark-ground illumination with low jiower, and with 

 high power a very white one, and its eftect is more secure ; I should 

 say the true one, in opposition to every other mode of illumination, 

 direct, oblique, with condensers or diaphragms. 



Its effect is particularly striking if you observe the flow of the 

 vegetable sap (rotary circulation in plants), as in the Tradcscantia 



