220 Proceedings of the Royal Irish Academy. 



It is quite clear that during the winter months, or those during 

 which the meridian sun's elevation above the horizon is low, sun- 

 light can penetrate into the church by both compartments of the 

 window, and show itself on the floor, or on the opposite wall, in the 

 form of two more or less rectangular parallelograms or patches of 

 light separated by a bar. 



It is also evident that for a certain meridional elevation of the sun 

 above the horizon, the upper portion of the window will not allow 

 any sunbeam to penetrate which could still penetrate by the lower 

 •one, and that finally for a still higher elevation of the midday sun — 

 that is, dui'ing the summer months — the meridian sun could send no 

 beams into the church by either part of this window. 



The dimensions that were taken of these openings allow of a 

 sufficiently close determination of what these different elevations may 

 be, and therefore of the periods of the year that would be indicated 

 by the appearance of both patches of light, of one only, or finally of 

 neither one nor other, on the floor or wall of the chui'ch, From the dia- 

 gram section herewith submitted (PL XVII., fig. 3), it maybe seen that 

 the angle of incidence of a beam of sunlight, for which it would cease to 

 penetrate, or would be " extinguished," in. the upper compartment of 

 the window, is 44° ; while in the lower part of the window the 

 corresponding angle is 52°. Now these angles of incidence of sunlight 

 would occur twice in the course of the year, for each compai'tment, 

 accordingly as the sun moves from one solstice to the other. For the 

 angle of 44°, the date would be 9th April and 4th September; while 

 for the angle of 52°, the dates are 2nd May and 11th August. (I 

 have here to acknowledge the kindness of Sir Eobert Ball, f.e.s., 

 of the Observatory, Cambridge, for these determinations.) 



It has already been pointed out that, taking into consideration the 

 direction of orientation of Dalkey Town church, the St. Begnet or 

 Bega to whom the church was dedicated was the virgin, venerated at 

 Dunbar, whose festival is given on the 6th September by Alban 

 Butler, and on the 3rd September by another authority. It may 

 therefore be assumed that the upper compartment of this window 

 was arranged so as to give notice or warning of the arrival of the 

 dedicatory saint's festival. 



It is probable that the under compartment of the window was 

 'intended to give some such warning or notice as regards some other 

 festival. 



Presuming, as has been already advanced, that the south-east 

 window of the Dalkey Island church was originally similar in style 



