476 Mr. C. H. Bennington on a New Photometer, 



eyeball to exclude stray lig'ht. F and E are the ends of B and 

 C. Let A be fixed in B at the distance to see x, and let the 

 time for photographing a house in full sunshine with a portrait- 

 lens of full aperture be already known. Hang a white blind over 

 one of the windows. Get the picture of the blind sharp in the 

 centre of the focusing-screen, and fix y on it by means of a pneu- 

 matic holder. Draw B nearly out of C, and, having cleared the 

 eye from impressions of light, make the observation. Push B in 

 till x is just distinct, and write Z on B where E now stands. 

 Divide FZ into six scales of XII., -XI., X., IX., VIIL, VII. 

 equal parts, counting I. in each scale from F. 

 The 7 and 12 scales will here do for examples. 



I I Seal 



(U W (I) 2 ($ (g ($ ©' U 



Z 



12\2 



(f (w m w m m m m ®* (W e-y ©; t 



Now, to take a darker subject with the same or a slower lens 

 and the same chemicals : — 



Choose a half-lighted portion of the subject, not the darkest 

 details nor the brightest, but a half light ; put against it, parallel 

 to the plate, enough white material to make a J -inch spot on the 

 focusing-screen. Fix the end y on the spot and make the ob- 

 servation ; to do this set E at I- x will perhaps be too distinct, 

 if so, turn E back till x is about as plain as before. Suppose E 

 happens to stand at IV. scale 12, then if a picture can be taken 

 in t (a known time) when x is distinct at Z, it can now be taken 



mtx yjy) =tx9. 



C should be lined with black velvet. B and C must be just 

 4 inches long, with x and y quite central, and the plane of x in 

 the end of B. 



The light, be it strong or weak, must be constant, or the test 

 fails. 



It may be said that as a visual index it does not measure 

 actinism ; but I assume that the amount of actinism is always 

 proportional to the amount of light, and J think that no prac- 

 tical error will occur if the observation be made in uncoloured 

 daylight; and with care and practice it is quickly made, which is 

 a very useful item in photometry. 



Melton, Suffolk, 

 November 6, 1867. 



Sea 

 12 



