and Remarks on the Action of Light, %c. 1S1 



Let Fig. 3 represent the same as Fig. 1, with this difference 

 only that°the stop or diaphragm S T, in front of the lens, is 

 smaller, so that a ray of light from the object at I could not 

 possibly fall on the centre of the lens, but half-way betwixt 

 the centre and the edge as at 1. From what has already been 

 said respecting Fig. 1, it will be evident that the error or dis- 

 agreement of focus must in this case be only the one-half of 

 what it is in Fig. 1, just as E 1 is half of E c. Hence, in 

 Paris or Italy, if, from clearness of the atmosphere and in- 

 tensity of the light, there be as much light refracted through 

 the lens in Fig. 3 as there is through that in Fig. 1 in this 

 country, the pictures taken there should have double the fine- 

 ness and sharpness of outline in comparison with ours, if their 

 lenses be equally good ; or, should they use the same dimen- 

 sion of stop or diaphragm as we do, then their picture should 

 double ours in force and brightness ; which I have no hesita- 

 tion in saying they would, if the process and apparatus were 

 the same. Having thus pointed out the disadvantages we 

 labour under in Daguerreotyping, I shall make a few observa- 

 tions on the taking of miniature portraits of the Daguerreo- 

 type. (Fig. 4.) 



It has already been observed, that, in taking large and flat 

 Daguerreotype views, the lens is placed in its worst position, 

 at least if we were going to use it as a telescope ; and in 

 taking miniature portraits the lens requires to be more like that 

 for a telescope than for a Daguerreotype, as in this case the 

 rays require to be rather concentrated in a small space than 

 spread over a large surface. The lens required for taking 

 large Daguerreotype views will do equally well for taking mi- 

 niature portraits. The only change required is to set the lens 

 in a double screwed cell, as shewn at EF, Fig. 4, and to screw 

 in the lens with its convex side to the object, the same as in 

 a telescope, when you are taking a miniature portrait, at the 

 same time removing the stop in front of the lens, and re- 

 placing it hy one of nearly double the diameter. In using the 

 lens fur large pictures, again, it must be screwed in with its 

 concave side towards the object, and the small stop must be 

 restored in front. The time required for taking a miniature 

 portrait will, by the use of the larger stop, and the consequent 



