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- TABLE DES MATIÈRES
- RECHERCHE DANS LE DOCUMENT
- TEXTE OCÉRISÉ
54
ROYAL SOCIETY OF CANADA
ascertained by trial. After development the screen is found to consist of semi-transparent and semi-opaque dots alternately disposed : examining it under a moderately strong microscope, the central part of the semitransparent dots must be thin, but not clear glass. The presence of minute spots of clear glass indicates under-exposure. Under a strong microscope it will be found that, even with correct exposure, there are very minute dots of clear glass in the centre of the semi-transparent dots ; their size must be kept down by a full exposure. A screen not sufficiently developed does not reproduce the deep shadows ; they come out solid black in the print, and the shadows are generally too dark. A screen too much developed gives a print in which the shadows are too light ; there is no solid black. Under-exposure is indicated by the absence of the finest white dots in the print. A long exposure, within reasonable limits, gives a dense screen, but correct gradations ; the only disadvantage is that it takes a long time to print. The collodion or emulsion must be of a kind giving soft negatives with good gradation. Special care must, of course, be taken to have the plate perfectly clean and everything in first class working order, so as to secure a screen free from defects.
In theory, the vignetted screen is the most perfect one for the photomechanical process, because correct prints may be obtained from thin or intense transparencies by using thin or intense screens, while in copying from a negative through a chess-board screen, the negative must be of right density to give a correct print.
Y. The Diaphragms.
The proper shapes for the apertures in the diaphragms, or “ stops.’5 as they are more generally called, have been described in the course of this paper; we have now to consider their adjustment.
The square is the only shape adapted to the cross-lined screen with unequal lines ; its adjustment has been given in equation (4) :
• _ iL
" uA
But this is not the proper wav to use a cross-lined screen. Its opaque and transparent lines should be of equal width, and it should be employed with double aperture stops, so as to work like a chess-board screen. The adjustment of the diaphragm is then given by the formula :
F
■ = or
F
A = :
Inj
The distance between the centres of the pair is :
e = J-v/27”
(16)
Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 98,27 %.
La langue de reconnaissance de l'OCR est l'Anglais.
ROYAL SOCIETY OF CANADA
ascertained by trial. After development the screen is found to consist of semi-transparent and semi-opaque dots alternately disposed : examining it under a moderately strong microscope, the central part of the semitransparent dots must be thin, but not clear glass. The presence of minute spots of clear glass indicates under-exposure. Under a strong microscope it will be found that, even with correct exposure, there are very minute dots of clear glass in the centre of the semi-transparent dots ; their size must be kept down by a full exposure. A screen not sufficiently developed does not reproduce the deep shadows ; they come out solid black in the print, and the shadows are generally too dark. A screen too much developed gives a print in which the shadows are too light ; there is no solid black. Under-exposure is indicated by the absence of the finest white dots in the print. A long exposure, within reasonable limits, gives a dense screen, but correct gradations ; the only disadvantage is that it takes a long time to print. The collodion or emulsion must be of a kind giving soft negatives with good gradation. Special care must, of course, be taken to have the plate perfectly clean and everything in first class working order, so as to secure a screen free from defects.
In theory, the vignetted screen is the most perfect one for the photomechanical process, because correct prints may be obtained from thin or intense transparencies by using thin or intense screens, while in copying from a negative through a chess-board screen, the negative must be of right density to give a correct print.
Y. The Diaphragms.
The proper shapes for the apertures in the diaphragms, or “ stops.’5 as they are more generally called, have been described in the course of this paper; we have now to consider their adjustment.
The square is the only shape adapted to the cross-lined screen with unequal lines ; its adjustment has been given in equation (4) :
• _ iL
" uA
But this is not the proper wav to use a cross-lined screen. Its opaque and transparent lines should be of equal width, and it should be employed with double aperture stops, so as to work like a chess-board screen. The adjustment of the diaphragm is then given by the formula :
F
■ = or
F
A = :
Inj
The distance between the centres of the pair is :
e = J-v/27”
(16)
Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 98,27 %.
La langue de reconnaissance de l'OCR est l'Anglais.