The common prism spectroscope is shown in figure is generally made up of three parts, viz., a prism and two telescopes. One of these telescopes is used to render the rays which fall on the prism parallel to each other, and is therefore called the collimator, the other telescope is made movable so that it can be turned into the proper direction for observing any desired color, and is, therefore, called the view-telescope. A circular scale is also provided to measure angular position of telescope. On the circular scale 29 divisions of the main scale exactly coincide with the 30 divisions of the vernier scale. The smallest division of the main scale is half-a-degree  $\left({0.5}^{\circ }\right)$. Initially view-telescope is focused to collect rays directly from collimator and position of telescope is measured. This is position (1). Now a prism is placed as shown in the figure and telescope is focused for red, yellow and violet. The respective positions of telescope are (2), (3) and (4).

	Reading of main scale	Reading of veriner scale
Position (1)	${20.0}^{\circ }$	20
Position (2)	${30.0}^{\circ }$	16
Position (3)	${32.5}^{\circ }$	9
Position (4)	${33.5}^{\circ }$	11

 
The deviation for: