124 CORRESPONDENCE. 



good specimens, as in Mailer's type slide, with a stop of an achromatic 

 condenser, limiting the illumination to a pencil of central rays of 20°. 

 Mr. Wenham's illuminator gives similar and more striking results. 



With Moller's slide I can examine the upper or lower surface, as 

 the glass of the slide is thin enough for a large-angled 5 inch of Ross 

 to work through, and that with the D eye-piece shows the framework 

 of the diatom to be composed of minute beads, whichever side is 

 uppermost. 



The irregularities, elevations, depressions, &c, are well worth 

 more attention than I have paid to them. 



Mr. Stewart — one of the best observers — made valuable remarks 

 when my paper was discussed, and he caused me to examine Coscino- 

 discus oculus Iridis in fractured specimens, and I found the depressions 

 real, and the hexagonal framework real. These depressed surfaces and 

 framework seemed, however, composed in the usual way, of minute 

 beads. 



I recommend to Mr. Wells' attention Mr. Stephenson's valuable 

 experiments of mounting fractured and whole diatoms in bisulphide 

 of carbon. 



I remain, Sir, your obedient servant, 



Henry J. Slack, 



Sec. R. M. 8. 



The Estimation of the Magnifying Power of Lenses. 



To the Editor of the ' Monthly Microscopical Journal.' 



Sir, — The world is advancing so fast in these days that people 

 ought to be prerjared for hearing anything ; especially people who 

 live, like me, in the mountains, and know they are behind the age. 

 Still, I own I was unprepared for what I saw to-day in your Journal. 

 When I was at school — a good time since — we used to do Algebra 

 and Optics ; not much, but some. And one of the things I remember 

 we learned was how to find the magnifying power of a lens. Now, I 

 see in this Journal of February, near the end of page 52, that this pro- 

 blem is claimed by Dr. Pigott, of Cambridge University. He says that 

 the person who has " worked it out " is himself. But if everyone knew 

 it long ago, how can it be that it is now discovered by him? It can 

 never be, surely, that the world has taken a turn the other way for a 

 change, and is going backwards instead of forwards ? Or, is it that, 

 like the rest of the age, lenses have got advanced ideas, and refuse to 

 magnify according to the former rules, as being too old-fashioned ? 

 This, no doubt, would call for a new solution, and might exjdain why 

 a learned Professor lately came to tell your Society that the old 

 Optics was now " played out." I was encouraged the more to think 

 it must be this, when I looked at the new " working out " ; for, if the 

 problem is old, the working out is quite new. He says, to get the 

 magnifying power, you divide 10 inches by the focal distance and 

 take one less. Very well, so I did. I took a 5-inch lens and, doing 

 this, there came out 1. So a 5-inch lens neither magnifies nor 

 diminishes, but acts like a piece of window-glass. Then I tried an 



