352 



KNOWLEDGE & SCIENTIFIC NEWS. 



[February, 1906. 



picture postcards, it will gfivc an cnlarijed image of any 

 papt-r print or diasjram, or of the page of a book without 

 injuring the hook in anv wav. The image as given 

 directly is, of course, laterally inverted, but where this 

 is a drawback, as in the case of figures or letters, this 

 is easily obviated by fixing a piece of silvered plate 

 glass (thin patent plate silvered on the back serves 

 well and is easily obtained in front of the lens and 

 receiving the reflected image on the screen. In addi- 

 tion to the ob\ious uses of such an apparatus, one can, 

 by its means, judge at once from a small print whether 

 an enlargement of it would be of advantage, and it will 

 often save the trouble of making a lantern slide and 

 arranging an optical lantern. 



TKe Glastonbviry Grace Cup. 



Peg-tank.\rds arc of the Saxon period. .\ fine speci- 

 men of .^nglo-Saxon work, formerly belonging to the 

 .\bbey of Glastonbury, is now in the possession of Lord 

 Arundel, the noble owner of Wardour Castle. This old 

 relic is known by the name of the "Glastonbury Cup, ' 

 as tradition sajs it was carved out of a piece of the 

 Holy Thorn on \\'eary-all Hill, when Joseph of Ari- 

 mathea rested and planted his staff, which, like Aaron's 

 rod, " budded and flowered." It is also known as the 

 "Grace Cup," and the old belief still exists that it is 



Grace Cup in Wardour Castle. 



the original Glastonbury Cup which was used as a 

 Grace Cup after meals in the refectory by the monks 

 of the .Abbey. It is made of wood, and rests on crouch- 

 ing lions, the bowl being carved w-ith the twelve 

 Apostles. On the lid, the Crucifixion is carved, with 

 the X'irgin Mary and St. John. Tlie Cup holds two 

 quarts, and originally had eight pegs placed one above 

 another inside, dividing the liquor into equal quan- 

 tities of half a pint each. 



The Rule for the Infinity 

 Focus. 



Bv ])R. (;. II. Hkva-W I'.k.S. 



Most instructions sent out with cameras contain some- 

 what dogmatic statements as to the distance.s at which 

 objects are in focus with a certain stop. It is sur- 

 prising that so few people know the verv simple rule for 

 such matters. 



Let a camera be focussed for " infinity," that is, for 

 parallel rays (as in the cheaper box cameras), and let it 

 be directed at a near object. Take two points on the 

 object, w hose distance apart is equal to the diameter ot 

 the stop. Then it is just possible to draw a couple of 

 parallel rays from the two points to the lens, and these 

 extreme rays will converge to the same point on the 

 negative. If the points are further apart, no rays from 

 one are parallel to rays from the other, nor do they 

 consequently converge to the same point on the nega- 

 tive. We thus have the following result : — In a fixed 

 focus camera the impressions produced on the plate or 

 film by two points of a near object will overlap if the 

 distance between the points is less than the diameter of 

 the stop. If the distance between the points is greater, 

 the impressions on the plate will be distinct. 



In most cameras the stop is placed between the lenses. 

 On looking through the front lens the stop appears 

 slightly magnified, and a little simple reasoning shows 

 that it is this magnified diameter, not the actual dia- 

 meter of the stop, that must be taken. 



For instance, if, on looking into the lens, the stop 

 appears to be half an inch in diameter, no objects less 

 than half an inch apart will produce separate im- 

 pressions; half an inch is, therefore, the limit of size 

 of the details which can be shown in the photograph. 

 It makes no difference whether the objects are near or 

 far away. If they are near, details below half an inch 

 in size may be large enough to appear blurred on tlie 

 negative, if they are far oft', such details may be un- 

 noticeable, but if the negative be enlarged to the same 

 size as before they will present the same appearance as 

 before, other circumstances being left out of account. 



-Another consequence of the rule may be noted. With 

 a lens of four inches focus, the limit of detail will be 

 one inch for stop f '4, and J inch for stop f/i6. With a 

 lens of one inch focus the limit of detail would be \ inch 

 for stop f/4, and i-i6th inch for stop f/i6. 



Suppose, then, that instead of taking quarter plate 

 negatixes with a four-inch lens, we were to take nega- 

 tives measuring about i inch by f inch with a lens of one 

 inch focal length and a corresponding stop, and suppose 

 we were to make enlargements of the same size from 

 both negatives. Then the enlargement from the 

 smaller negative would show four times the detail in 

 focus at all distances; that is, details of I inch would 

 be in as good focus in the enlargement of the smjiller 

 negative as details of one inch in that of the larger. By 

 c;u-rying a very tiny camera about in his watch pocket 

 and trusting to enlarging, a photographer might be 

 practically independent of focussing even for very near 

 objects indeed, but for the fact that imperfections and 

 coarseness of grain in the negatives would be enlarged 

 correspondingly. 



