TJie Achromatic Telescope. 179 



by morning for weeks myself and a servant hunted them 

 out, and it is not too much to say that we often took 

 from twenty-five to thirty of these intruders from a single 

 plant, although the day before we had thought there could not 

 be one left, so careful had been our search. I had been proud 

 of my ferns, but my pride was sadly humbled; and all unwilling 

 as I am to kill anything, I was obliged to allow these devourers 

 to be crushed. 



Last summer again the same thing happened. They came 

 in incredible numbers. Before I was aware, they had again 

 despoiled my Athyriums of every vestige of green, and in 

 order to save my remaining ferns, we were compelled again to 

 perpetrate a daily massacre. 



Their plan was, each year, to attack the Athyriums first, then 

 Lastrea filiz-mas , after that the other Lastreas, then the Eoyal 

 Osmunda, and when these failed, but not till then, they were 

 not too dainty to try Polystichum lobatum. The Polystichums 

 were always the last in requisition, they evidently preferred 

 something more tender. I hope next summer they may take 

 it into their heads to commit their depredations elsewhere than 

 in my garden. 



THE ACHROMATIC TELESCOPE, DIALYTES, AND 



FLUID LENSES— NEBULA— DOUBLE STARS— 



OCCULTATIONS. 



BY THE EEV. T. W. WEBB, A.M., P.R.A.S. 



In several previous papers we have endeavoured to give a 

 tolerably comprehensive idea of this beautiful invention ; but 

 a. few particulars, not unworthy of notice, yet remain. 



The chief impediment to the manufacture of object-glasses 

 of sufiicient aperture to compete with the light-grasping 

 power of large reflectors, used to be the want of homogeneous 

 flint-glass ; the salt of lead which is employed in its manufac- 

 ture, and which gives it its superior dispersive power, having 

 a tendency from its weight not to mix uniformly with the 

 other ingredients, and if intermingled by mechanical means, 

 to form streaks and veins very prejudicial to neat definition. 

 Small discs might easily be procured, but with increasing 

 size all kinds of imperfections increased with fatal rapidity, so 

 that, as recently as 1820, lenses of five or six inches were 

 scarcely procurable. In order to obviate this serious evil a 

 remarkable modification was devised in the year 1828, by Sir 

 VOL. VII. — no. in. N 



