£fj PERISCOPIC CAMBRA AND MICROSCOPE. 



petunlly moist during hot sunshine, by water dribbling through 

 the soil to their roots, without wetting their leaves, which, im- 

 mediately evaporating by the heat, will cool the air just above 

 them. In fact, it is only by a close imitation of the process of 

 nature, that these vegetables of cold regions can be successfully 

 cultivated in botanic gardens. 



They must be The last essential nont relative to a/pine plants is to cover 



covered from , r . / r 



frost. them up on the approach of frost : this may appear a strange 



precaution to some, bin when winter commences in their native 



• soil, being immediately covered with snow to the depth of seven 



inches, they never feel a greater degree of cold than that of the 



freezing ppint, the soil itself being hardly frozen. The best 



covering is that of fern, pteris aquilina, which does not absorb 



moisture so quickly as most other sorts of haulm. 



V. 



On a Periscopic Camera Ohscura and Microscope. By Wil- 

 liam Hyde Wollaston, M. D. Sec. R, S. From the 

 Philosophical Transactions for 1812, p. 370. 



Periscopic im- A LTHOUGH the views which I originally had of the ad- 



provement of x^L vantage to be derived from the periscopic construction of 

 the camera. ° i 



spectacles*, naturally suggested to me a corresponding improve- 

 ment in the camera ohscura, by substituting a meniscus for the 

 double convex lens, I have hitherto deferred making it known 

 to others, except as a subject of occasional conversation. 

 Themathema- Since in vision with spectacles, as in common vision, the 



ti^al conside- pencl i f r ays received by the eye in each direction is small, 



ration applied 1 " . f . '. * / . . 



to spectacles is the superiority of that form or glass, which disposes all parts 



not with con- f ft m0s t nearly at right angles with the visual ray, admits of 

 plicable to the distinct demonstration j but with respect to the camera ob- 

 cam. ob8. scura, where the portion of lens requisite for sufficient illumi- 

 nation, is of considerable magnitude, although it is evident 

 that some improvement may be made in the distinctness of 

 oblique images on the same principles, yet as the focus of 

 oblique rays is far from being a definite point, the degree in 

 which it may be improved is not a fit subject of mathematical 

 investigation. 



* Phil. Magaz. Vol. XVII. Nicholson's Journal, VII. 143. 



I have 



