Solar plate measuring machine or solar micrometer. The rectangular wooden base plate holds the circular brass plate at an angle tilted towards the user. Across the circular brass plate is a moveable eyepiece on a fixed arm. This instrument was used towards the end of the 19th century and well into the 20th century at the Observatory to measure the position of sunspots on glass plate negatives taken of the sun. These photographs were taken daily at the Observatory (weather permitting).