Dr Brewster's Description of a Teinoscope. 355 
lineal proportions of objects. By moving one of the prisms in 
the plane of refraction, so as to go on each side of the achromatic 
position, the proportion of the length to the breadth of the ob- 
ject was altered, though the colour was of course not entirely 
corrected. 
In order to obviate this inconvenience, I contrived a com- 
pound instrument of four prisms, as represented in Plate X. 
Fig. 9.5 where AB is a prism with a variable refracting angle, 
as used by Clairaut for measuring, dispersive powers. It consists 
of two prisms A, B, of the same material, united by two cylin- 
drical surfaces c d ef so that by the motion of the concave cy- 
lindrical surface of B upon the convex one of A, the two plane 
surfaces MN form different angles with one another. Another 
prism C, of the same glass, is placed as in the figure, so as to 
correct the colour of AB, when the inclination of M and N has 
its mean value. 
If we now look at any object, through the faces m n, M, N, it 
will be quite free of colour, and its length (if in the plane of re- 
fraction,) will be magnified or extended, so that we can jndge 
whether its form would be improved by varying its longitudinal 
dimensions. If we wish to examine its appearance when its 
length is made greater or less in proportion to its breadth, we 
have only to enlarge or diminish the inclination of the faces 
MN, and then correct the colour, by placing the prism C at a 
greater or a less distance from M. In this way we may ascer- 
tain the exact relation between the length and breadth of an ob- 
ject, which is most agreeable to the eye, in place of discovering 
it by a succession of designs. 
As the effect of the different designs, however, possessing dif- 
ferent ratios of length and breadth, could not be recollected so 
as to be brought into direct comparison, it became advisable to 
have two instruments adjusted to give the proportions which it 
was required to compare ; and by combining the two instru- 
ments together, we obtained the additional advantage of enlar- 
ging the extending power of each instrument, when they were 
placed similarly or in the same plane ; and of obtaining a varia- 
tion in the breadth of the object when they were placed trans- 
versely or in rectangular planes. By such means we had it in 
our power to vary either the lengtli or breadth of the object. 
