You might notice a height difference between the old and new shots. There’s a good reason for that. When the building was constructed as Illinois’ 5th Statehouse in 1839 it was approximately the same height it is today. Around the turn of the 20th century, while the building was being used as the Sangamon County Courthouse, it was raised and had an extra floor added – which you can see in the 1938 photo.
In 1966, there was an extensive reconstruction that removed that extra story and added below-ground parking. You can read more about that project at Look Back Springfield.